JMA reported that, based on seismic data, the eruption from Asosan’s Nakadake Crater that began on 25 November 2014 continued during 27 April-1 May. Ashfall was reported in areas to the SE and NE on 27 April. Field surveys were conducted on 27 April and 1 May; observers noted an off-white plume rising from the vent. A plume rose as high as 1.2 km above the crater rim on 29 April. The Alert Level remained at 2 (on a scale of 1-5).

Ongoing emissions leading to a larger, late-2014 eruption with ashfall

This report summarizes behavior at Naka-dake (Nakadake) crater at Asosan (Aso, Aso-san) caldera chiefly during January 2014-February 2015. During this reporting interval Naka-dake continued to emit gas, steam, and small ash plumes. A larger eruption took place starting 25 November 2014, causing ashfall and glowing emissions. This closed a local airport, and triggered hundreds of reports on ash plumes for the aviation community by the Tokyo Volcanic Ash Advisory Center (VAAC). That eruption continued through 2014. The eruption went on into 2015 but was generally described as intermittent during late December 2014 through at least the end of February 2015. The Alert Level remained at 2 (on a scale increasing from 1 to 5) for the duration of the reporting interval. Our last report, BGVN 37:08, described the emission of ash plumes and other behavior during May-June 2011. Some remarks in this report also refer to earlier behavior, for example, a short subsection includes what JMA recorded as important in a terse summary of 2011.

Eruption details were extracted and synthesized mainly from Japan Meteorological Agency (JMA) sources. JMA frequently communicated with the Tokyo VAAC about Asosan’s eruptive status. This report also discusses Volcano Ash Advisories (VAAs) issued by the JMA’s Tokyo VAAC. For many of the VAAs, evidence of ash-bearing plumes reported by JMA could not be reliably detected in the satellite images. For example, the images were sometimes obscured by overlying weather-cloud cover. The plumes were also generally only rising to a few kilometers in altitude. In at least some cases, the low plumes appeared bent by high winds.

Naka-daka Crater Number 1 remained the active vent for the most part during the past eight decades. That same pattern held true during this reporting interval when myriad small eruptions, often to or below 1 km above the crater rim were documented. Visibility was sometimes impaired but monitoring instrumentation confirmed a pattern of ongoing eruption. In some cases, the eruption was not clearly seen but fresh ash was recorded. Webcameras regularly documented incandescence both in the crater and onto the crater rim. Smaller ash plumes were too numerous to mention except in occasional cases. High winds were often mentioned, which may have bearing on restricting plume heights.

Location and brief background. Asosan is located on the S of the main island of Japan (Honshu) on the island of Kyushu (figure 32).

The rim of Nake-dake is unusually developed for such an active volcano. Both a road and cable car carry tourists there. Shelter dugouts are provided around the crater. The Aso Volcano Museum is located nearby.

Figure 33, made from radar imagery, shows Asosan’s morphology.

Figure 33. Morphology of the Asosan caldera seen in shaded relief (color scale for elevations absent). N is towards the top; the N-S cross-caldera distance is ~25 km. Note the central highlands, a series of ~17 post-caldera cones that includes the active Nake-dake and its Crater Number 1. The caldera’s topographic boundary is distinct on all sides but a drainage has breached the W side. This caldera vented four sets of massive eruptive deposits (pyroclastic-flows and associated ash-falls; Miyabuchi, 2013; Fujii, 2001). Source: Wikipedia (from a Nasa Shuttle Radar Topography Mission).

JMA (2013) includes a map showing the location of 12 calderas in Japan. Asosan, the largest and most active, has had many small eruptions in the past few thousand years, including many witnessed eruptions in the interval of recorded history. Spica (2013) discusses Aso in the context of other calderas in the Kyushu region.

Figure 34 shows a shaded-relief map focusing on the post-caldera cones in the central highland area.

Figure 34. A shaded relief map of the elevated central area (post-caldera cones and their craters) of the Aso caldera, adding naming labels in English of some of the main features. As seen in the previous figure, the caldera floor (moat) is outside and encircling this central topographic high. Both conventional topographic and a digital elevation map (50 m grid) were used to make this map, which was published by the Geospatial Information Authority of Japan. Source: JMA (2013).

JMA’s website features this summary on Asosan.

“Asosan (Aso Volcano) comprises the Aso caldera and post-caldera central cones. The Aso caldera, 25 km north-south and 18 km east-west in diameter, was formed by four gigantic pyroclastic-flow eruptions from approximately 270,000 to 90,000 years ago. Post-caldera central cones were initiated soon after the last caldera-forming eruption, producing not only local lava flows but also voluminous tephra layers which fell far beyond the caldera. Nakadake Volcano, which is the only active central cone of basaltic andesite to basalt [composition], is one of the most active volcanoes in Japan. The active crater of Nakadake Volcano is a composite of seven craterlets aligned N-S [elongate zone of depressions to the left of the label “Nakadake” and above the letter ‘t’ in the label “Nakadake-Crater” on figure 34; see also SEAN 04:07 for a sketch map focused on this area)]. Only the northernmost [Nakadake] crater (No. 1 crater) has been active in the past 80 years, although some others were active before the 1933 eruption. The Nakadake No. 1 crater is occupied by a hyperacidic crater lake during its calm periods. During active periods, its volcanic activity is characterized by ash and strombolian eruptions and phreatic or phreatomagmatic explosions.”

According to Fujii and others (2001), “Aso caldera in central Kyushu, Japan, is one of the largest calderas in the world and covers an area of 380 km2. In late Pleistocene time, eruptions of voluminous pyroclastic flows occurred intermittently, resulting in formation of the caldera. The Aso pyroclastic-flow deposits are divided into four major units, i.e. Aso-1, Aso-2, Aso-3, and Aso-4 . . . [and] welded tuffs of these units are widely distributed in central Kyushu, and are generally well suited for paleomagnetic research . . .. K-Ar ages for Aso-1, Aso-2, Aso-3, and Aso-4 have been determined to be 266 ± 14 ka, 141 ± 5 ka, 123 ± 6 ka, and 89 ± 7 ka, respectively (Matsumoto and others, 1991).”

JMA summary for 2011 activity. JMA (2013) tabulated a summary of witnessed events (eruptions, possible eruptions, damage, significant behavior, etc.) at Asosan going back to the year 553. In the most recent behavior discussed, the authors briefly note that during 2011 (an interval they term Heisei 23) the following behavior occurred.

First, after the Mw ~9 Tohoku earthquake ~70 km off the Pacific coast on 11 March 2011, earthquakes temporarily increased roughly 10 km to the NW of the active crater. Second, very small emissions of gray-white volcanic ash occurred during 15 May to 9 June 2011. On 15 May a very small amount of tephra fall was confirmed at Sensuikyo, ~2 km to the NE of the Nakadake Number 1 crater.

2014 Activity. JMA reporting for 13 January 2014 noted the emission of a very small eruption. This came in the wake of increased tremor in late December 2013 and an increase in hazard status to Alert Level 2. (As previously mentioned, the Level remained at 2 for the duration of the reporting interval.) Further escalation in tremor took place on 2 January. On 10 January emissions reached 1,200 metric tons/day (t/d) of sulfur-dioxide (SO2). The 13 January 2014 eruption took place at Naka-dake, which emitted a grayish-white plume that rose to 600 m that traveled S and deposited traces of ash. The resulting report from the Tokyo VAAC (a Volcanic Ash Advisory (VAA)) stating they failed to detect identifiable ash in the plume data captured on satellite images.

The small 13 January 2014 eruption triggered the first Asosan VAA in over a year. The other VAAs during January 2014 were issued on the 27th, 29th, 30th, and 31st. On one of those days, two VAAs were issued, and thus, for January there were 6 VAAs.

Bulletin editors note that the VAAs are not a linear measure of the number of eruptions. Small eruptions may not trigger a VAA at all. Several consecutive VAAs may occur associated with a single potentially larger eruption, which are issued in an effort to track an ash plume. Again, this may be an example where the number of VAAs is not reflective of the number of eruptions. Despite this, the number of VAAs are easily counted owing to new online archives. The Tokyo VAACs online presentation system is tablular in nature and is thus well suited to enable a count of reports per month.

The tally for VAAs on Asosan during 2013 was zero. The tally for 2014 involved 171 VAAs. Monthly totals for 2014 are as follows: January, 6; February, 3; March-July, 0; August, 3; September, 2; October, 0; November, 25; and December, 132. For further comparison, the tally for January and February 2015 involved 250 VAAs, with January, 132, and February, 118.

JMA reported that seismicity increased from 21 to 23 January 2014, and then decreased on 24 January. On 23 January a volcanologist observed ash plumes rising from the central vent on the crater floor. On 29 January an ash plume reported by a pilot rose to 2.7 km altitude and drifted NW. Later that day a plume rose to an altitude of 1.5 km and drifted N. JMA reported that a very small Asosan explosion occurred on 31 January. An off-white plume rose 100 m above the crater rim and drifted S.

According to the Tokyo VAAC during 30 August-1 September 2014 eruptions continuously emitted ash plumes that rose to heights of 1.2-2.1 km drifting N and NE. For example, on 1 and 6 September eruptions emitting trace amounts of ash sent plumes 600 m above the rim. (Tokyo VAAC issued VAAs stating this plume lacked identifiable ash in available satellite images.) JMA instrument surveys established SO2 flux rates on 21 August of 1,000 t/d, and in early September of 1,200 tons/day. Counts tallying daily volcano-tectonic earthquakes (and cases of tremor) were made during 1-4 September occurring in the range 48-92 (429-500); during 5-7 September occurring the in the range 55-129 (401-463); during 8-15 September occurring in the range 394-564 (80-174).

JMA reported that during 8-16 September a persistent white plume was observed 1 km above the crater.

Preliminary counts for volcanic earthquakes (394-564 per day) and tremor (80-174 per day) were reported during 8-15 September. Field surveys conducted on 9 and 12 September yielded elevated temperatures from fumaroles and the surface of the S crater wall.

Tremor accompanied a very small eruption recorded on 22-24 October. Ashfall observed on the 24th indicated another such eruption.

During 7 September and 24 November 2014, VAAs were absent for Aso. In contrast, during 25 November 2014-31 December 2014 there were 171 VAAs issued. Multiple VAAs were issued on several different days in this later interval, for example, on 26 November, 7 VAAs were issued.

Asosan continued to erupt during the 7 September-24 November 2014 interval. Some monitored parameters such as earthquakes, tremor, and SO2 emissions were elevated. A small eruption took place on 6 September, for example, sending a plume to 600 m above the crater. During 8-16 September JMA noted a persistent white plume 1 km above the crater. During the week 12-18 November, a steam plume rose 400 m above the crater rim.

With the start of the surge in VAAs beginning on 25 November 2014 (noted above), a stronger and comparatively sustained eruption began. During the eruption on the 25th an ash plume rose to 1.8 km above the crater rim. Ash soon fell to the E in Hanoi Aso (Kumamoto Region), Taketa (30 km NE, Oita Region), Gokase (25 km WSW, Miyazaki Region), and in Minamiaso (10 km SW, Kumamoto Region). Incandescence at night was seen with webcams.

On 26 November tephra ascended 100 m above the crater rim and an ash plume rose 1 km. Tremor began a few hours before the eruption and on the 26th, continued to be elevated. The eruption continued on 27 November; ash plumes rose 1.5 km. Volcanologists observed a strombolian eruption and found 7 cm of fresh ash that contained fist-sized scoria. Ash fell to the W, affecting the city of Kumamoto (38 km WSW). According to a news article, flights in and out of Kumamoto airport were either cancelled or diverted. On 28 November ash plumes rose 1.5 km. The eruption continued through at least 30 November; ash plumes rose at most 1.5 km and incandescent material was ejected onto the crater rim.

Although inclement weather restricted views of the crater, monitored parameters and available views indicated that the 25 November eruption continued through to at least 22 December, when it became intermittent. Ash plumes to about 1 km above the crater rim and incandescent material on the crater rim were common through the end of the year (and beyond, through this reporting interval ending in February 2015, and described as the ongoing eruption.

A news report in the 28 November 2014 issue of the Daily Mail by Sara Malm (Malm, 2014) indicated dozens of cancelled flights at Kumamoto’s airport. That report included the Associated Press photo seen in figure 35. The date of the photo in that article was ambiguous, but a different article with the same photo (see caption) gave 26 November 2014 as the photo’s date. The angled, bent-over character of the ash plume and location of Crater Number 1 (the active crater, at the N end of the row of craters) indicate the view was from the NW and implies strong winds roughly from the N.

Figure 35. A photo taken on 26 November 2014 of Asosan in eruption. The gray ash plume is escaping at Nake-dake Crater Number 1 blowing roughly S. The plume does not rise vertically. The plume ascends near the vent but for some distance beyond the vent the plume descends. At distance the plume appears to spread over considerable vertical extent, from near the ground surface to above the field of view. Source: Phys.Org news (crediting AP/Kyodo News).

An undated video in Malm (2014) also showed the plume. The video also showed an aerial view of the visitor area on the crater rim, which was blanketed in gray ash. Other scenes included children walking to school wearing dust masks and carrying folded umbrellas, and close up shots of what appeared to be dark colored, highly vesicular spatter.

In a 29 November 2014 MODIS image of the region, Asosan was under weather clouds but a clear view revealed a prominent ~30-km-long, beige-colored, funnel-shaped area trending SE. This was interpreted by Nasa Earth Observatory authors Jeff Schmaltz and Adam Voiland as airborne ash. Webcamera images around this time showed a glowing pit crater with extensive areas containing incandescent tephra around it. A copious plume also discharged nearby.

During a field survey on 10 December volcanologists observed 20-cm-wide blocks near the crater and 5- to 10-cm-wide blocks within 1.2 km SW of the crater. During 12-15 December the plume rose 1 km above the crater rim and ash fell to the E in Hanoi Aso (Kumamoto Region).

JMA reports for 15-30 December described the usual eruptive ash plumes that again rose 600-1,000 m above the crater and some cases of still glowing material on the crater rim. SO2 fluxes were 2,000-3,100 t/d during 15 and 18 December.

2015 activity. As noted above, the VAAs for 2014 totaled 171, and the VAAs for the months of January and February 2015 totaled 250. This is consistent with ongoing eruption at Asosan, which was also the basic conclusion in JMA reports from monitoring and direct observations during January-February 2015, although they often described the eruption during both these months as intermittent.

JMA reported cases during January where plumes rose up to 1 km above the crater, and in some cases glowing material reached the crater rim. JMA reported SO2 fluxes of 500-2600 tons of SO2. Both tilt and GPS instrumentation recorded slight growth across the active crater. A pilot report on 29 January indicated an ash plume to 2.7 km altitude (~1.1 km above the rim) and drifting NW.

An image acquired on 13 January 2015 was discussed by Jesse Allen and Adam Voiland of Nasa Earth Observatory. They reported that the image was from the Operational Land Imager (OLI) on Landsat 8. They indicated that it showed ash drifting ten’s of kilometers S from Aso.

For February 2015, JMA reported episodes of volcanic earthquakes, high-amplitude tremor, and infrasound data that continued to indicate ongoing intermittent eruptions. Webcamera views again documented cases of glowing material reaching the rim during the first half of the month. Plumes again rose up to 1 km above the crater rim. JMA reported intermittently detected eruptions, including during 2-6, 9-13, and 16-20 February.

Malm, S, 2014, Flights cancelled across Japanese region after Mount Aso volcano erupts for the first time in 22 years, spewing lava, smoke and a kilometre-high ash cloud, The Daily Mail 28 November 2014 (7 graphics files and a 58-second video) (accessed online June 2015) ((URL: http://www.dailymail.co.uk/news/article-2852674/Volcano-south-Japan-erupts-disrupting-flights.html#ixzz3d5POqZhu )

JMA reported that, based on seismic data, the eruption from Asosan’s Nakadake Crater that began on 25 November 2014 continued during 27 April-1 May. Ashfall was reported in areas to the SE and NE on 27 April. Field surveys were conducted on 27 April and 1 May; observers noted an off-white plume rising from the vent. A plume rose as high as 1.2 km above the crater rim on 29 April. The Alert Level remained at 2 (on a scale of 1-5).

JMA reported that, based on seismic data, the eruption from Asosan’s Nakadake Crater that began on 25 November 2014 continued during 20-24 April. A plume rose 1.5 km above the crater rim and incandescence from the vent was observed at night. Field surveys confirmed that ash emissions from the active vent rose as high as 200 m above the crater rim. High-amplitude tremor continued. The Alert Level remained at 2 (on a scale of 1-5).

JMA reported that, based on seismic data, the eruption from Asosan’s Nakadake Crater that began on 25 November 2014 continued during 30 March-3 April. A plume rose 900 m above the crater rim and incandescence from the vent was observed at night. Field surveys confirmed that rumbling originated from the active vent. High-amplitude tremor continued. The Alert Level remained at 2 (on a scale of 1-5).

JMA reported that, based on seismic data, the eruption from Asosan’s Nakadake Crater that began on 25 November 2014 continued during 13-16 March. High-amplitude tremor continued, although it had decreased on 9 March. The Alert Level remained at 2 (on a scale of 1-5).

JMA reported that, based on seismicity and infrasound data, the eruption from Asosan’s Nakadake Crater that began on 25 November 2014 continued during 2-6 March. Incandescent material was sometimes ejected onto the crater rim and plumes rose 1 km above the crater. High-amplitude tremor continued to be detected and infrasound data sometimes indicated eruptions. The Alert Level remained at 2 (on a scale of 1-5).

JMA reported that, based on seismicity and infrasound data, the eruption from Asosan’s Nakadake Crater that began on 25 November 2014 continued during 16-20 February. Plumes rose 900 m above the crater and high-amplitude tremor continued to be detected. The Alert Level remained at 2 (on a scale of 1-5).

JMA reported that, based on seismicity and infrasound data, the eruption from Asosan’s Nakadake Crater that began on 25 November 2014 continued intermittently during 9-13 February. Incandescent material was sometimes ejected onto the crater rim, and plumes rose 600 m above the crater. High-amplitude tremor continued. The Alert Level remained at 2 (on a scale of 1-5).

JMA reported that, based on seismicity and infrasound data, the eruption from Asosan’s Nakadake Crater that began on 25 November 2014 continued intermittently during 2-6 February. Incandescent material was sometimes ejected onto the crater rim, and plumes rose 1 km above the crater. High-amplitude tremor continued. The Alert Level remained at 2 (on a scale of 1-5).

JMA reported that, based on seismicity and infrasound data, the eruption from Asosan’s Nakadake Crater that began on 25 November 2014 continued intermittently during 26-30 January. Incandescent material was sometimes ejected onto the crater rim, and plumes rose 800 m above the crater. High-amplitude tremor continued. The Alert Level remained at 2 (on a scale of 1-5).

JMA reported that, based on seismicity and infrasound data, the eruption from Asosan’s Nakadake Crater that began on 25 November continued intermittently during 19-26 January. Incandescent material was sometimes ejected onto the crater rim. Plumes rose 1 km on 20 January and 600 m each day during 23-26 January. The Alert Level remained at 2 (on a scale of 1-5).

JMA reported that, based on seismicity and infrasound data, the eruption from Asosan’s Nakadake Crater that began on 25 November continued intermittently during 5-9 January. On 5 and 7 January incandescent material was sometimes ejected onto the crater rim. On 7 January plumes rose 1 km above the crater. The Alert Level remained at 2 (on a scale of 1-5).

JMA reported that, based on seismicity and infrasound data, the eruption from Asosan’s Nakadake Crater that began on 25 November continued intermittently during 22-26 December. Plumes rose 1 km above the crater and incandescent material was sometimes ejected onto the crater rim. The Alert Level remained at 2 (on a scale of 1-5).

JMA reported that, based on seismicity and infrasound data, the eruption from Asosan’s Nakadake Crater that began on 25 November continued during 15-22 December. Plumes rose 600-1,000 m above the crater and incandescent material was sometimes ejected onto the crater rim. Sulfur dioxide emissions were 2,000-3,100 tons/day during field observations on 15 and 18 December. The Alert Level remained at 2 (on a scale of 1-5).

JMA reported that, based on seismicity and infrasound data, the eruption from Asosan’s Nakadake Crater that began on 25 November continued during 8-12 December; inclement weather often prevented observations of the crater. A webcam recorded tephra being carried by high winds and deposited in an area 1 km W. During a field survey on 10 December volcanologists observed 20-cm-wide blocks near the crater and 5-10-cm-wide blocks within 1.2 km SW of the crater. Plumes rose 600 m above the crater and incandescent material was sometimes ejected onto the crater rim. During 12-15 December the plume rose 1 km above the crater rim. Ashfall was reported E in Hanoi Aso (Kumamoto Region). Incandescent material was occasionally ejected onto the crater rim. The Alert Level remained at 2 (on a scale of 1-5).

JMA reported that, based on seismicity and infrasound data, the eruption from Asosan’s Nakadake Crater that began on 25 November continued during 1-8 December; inclement weather often prevented observations of the crater. A webcam recorded incandescent material being ejected from the crater at night. A very small amount of ashfall was observed at Sensui Gorges, 2 km NE on 3 December. The Alert Level remained at 2 (on a scale of 1-5).

JMA reported that on 25 November an eruption from Asosan’s Nakadake Crater occurred after increased tremor detected a few hours earlier. Ash plumes rose from the crater and produced ashfall to the E in Hanoi Aso (Kumamoto Region), Taketa (30 km NE, Oita Region), Gokase (25 km WSW, Miyazaki Region), and in Minamiaso (10 km SW, Kumamoto Region). Incandescence from the crater was recorded at night with webcams. On 26 November tephra was ejected 100 m above the crater rim and an ash plume rose 1 km. Tremor continued to be elevated. The eruption remained strong on 27 November, and ash plumes rose 1.5 km. During a field survey in an area S of Nakadake Crater volcanologists observed Strombolian activity in the crater, 7 cm of ash deposition, and fist-sized scoria. Ashfall was reported in a wide area to the W, mainly in Kumamoto (38 km WSW). According to a news article, flights in and out of Kumamoto airport were either cancelled or diverted. On 28 November ash plumes rose 1.5 km. The eruption continued through 30 November; ash plumes rose at most 1.5 km and incandescent material was ejected onto the crater rim. Inclement weather mostly prevented views of the crater during 1-2 December, but the small-scale eruption likely continued. The Alert Level remained at 2 (on a scale of 1-5).

JMA reported that Alert Level 2 at Asosan continued during 10-14 November. A white plume rose 400 m above the crater. Incandescence from Nakadake Crater was visible at night. The Alert Level remained at 2 (on a scale of 1-5).

JMA reported that a period of tremor from a very small eruption at Asosan was recorded late at night on 22 October through early the next morning. The video camera near the crater had ash adhered to it. During a field survey on 24 October volcanologists observed ashfall around Nakadake Crater. The Alert Level remained at 2 (on a scale of 1-5).

JMA reported that Alert Level 2 at Asosan continued during 29 September-3 October. A persistent white plume rose 600 m above the crater. Incandescence from Nakadake Crater was visible during 29-30 September. The Alert Level remained at 2 (on a scale of 1-5).

JMA reported that Alert Level 2 at Asosan continued during 8-16 September. A persistent white plume was observed 1,000 m above the crater.

Preliminary counts for volcanic earthquakes (394-564 per day) and tremor (80-174 per day) were reported during 8-15 September. Field surveys conducted on 9 and 12 September yielded elevated temperatures from fumaroles and the surface of the S crater wall.

JMA reported that Alert Level 2 at Asosan continued during 1-8 September. A small eruption occurred on 1 September from Nakadake crater, generating an off-white plume that suggested a small amount of ash; the plume rose 1,200 m above the crater. Incandescence from the crater was detected with a camera on 2 September. Volcanic earthquakes (48-92 per day) and tremor (429-500 per day) was detected during 1-4 September.

On 6 September a small eruption occurred from Nakadake crater that generated a plume 600 m above the rim. Elevated SO2 (1,200 tons/day) was detected during a field survey (the previous measurement on 21 August was 1,000 tons/day). Volcano-tectonic earthquakes (55-129 per day) and tremor (401-463 per day) was detected during 5-7 September.

Tokyo VAAC issued advisories based on JMA reports of eruptions on 1 and 6 September, though no volcanic ash was visible in satellite images.

On 27-29 August JMA reported volcanic earthquakes and elevated lake temperatures at Asosan's Nakadake Crater. On 30 August an eruption occurred with a gray-white plume of indeterminate height due to clouds prompted raising the Alert level to 2. On 31 August- 2 September was a small eruption and a gray white plume rose 800-1200 m above the crater rim. During 30 August-1 September the Tokyo VAAC reported ash plumes that rose 1.5-2.1 km (5,000-7,000 ft) a.s.l. and drifted N and NE. The Alert Level was raised to 2 (on a scale of 1-5).

JMA reported that a very small explosion from Asosan's Nakadake Crater occurred on 19 February. An off-white plume rose 200 m above the crater rim and drifted SW. During fieldwork on 21 February volcanologists noted that sulfur dioxide emissions remained high. The Alert Level remained at 2 (on a scale of 1-5).

JMA reported that a very small explosion from Asosan's Nakadake Crater occurred on 16 February. An off-white plume rose 300 m above the crater rim and drifted S. The Alert Level remained at 2 (on a scale of 1-5).

According to a JMA report, volcanologists conducting a field survey of Asosan's Nakadake Crater on 5 February detected decreased sulfur dioxide emissions and fewer volcanic earthquakes. The Alert Level remained at 2 (on a scale of 1-5).

Based on pilot observations, the Tokyo VAAC reported that on 29 January an ash plume from Asosan rose to an altitude of 2.7 km (9,000 ft) a.s.l. and drifted NW. Later that day a plume rose to an altitude of 1.5 km (5,000 ft) a.s.l. and drifted N. JMA reported that a very small explosion from Naka-daka Crater occurred on 31 January. An off-white plume rose 100 m above the crater rim and drifted S. The Alert Level remained at 2 (on a scale of 1-5).

JMA reported that seismicity at Asosan increased from 21 to 23 January, and then decreased on 24 January. On 23 January a volcanologist observed ash plumes rising from the central vent on the crater floor. The Alert Level remained at 2 (on a scale of 1-5).

On 27 December 2013 JMA raised the Alert Level for Aso to 2 (on a scale of 1-5) because volcanic tremor amplitude had been increasing since 20 December. However, on 2 January 2014 the amplitude rapidly decreased. Sulfur dioxide emissions were 1,200 tons per day during 2-9 January and 1,500 tons on 10 January. Volcanic tremor amplitude increased between 0800 and 1900 on 12 January. At 1215 on 13 January a very small eruption from Naka-dake Crater generated a grayish white plume that rose 600 m and drifted S, producing ashfall downwind.

Based on notices from JMA, the Tokyo VAAC reported that during 18-22 May ash plumes from Aso rose to altitudes of 1.5-2.1 km (5,000-7,000 ft) a.s.l. and drifted N, NE, and SE. A pilot noted on 18 May that an ash plume rose to an altitude of 3 km (10,000 ft) a.s.l.

Based on pilot observations, the Tokyo VAAC reported that on 15 May an ash plume from Aso rose to an altitude of 2.1 km (7,000 ft) a.s.l. and drifted NE. JMA reported that the next day plumes rose to altitudes of 1.8-2.1 km (6,000-7,000 ft) a.s.l. A pilot noted that an ash plume rose to an altitude of 2.4 km (8,000 ft) a.s.l. and drifted N that same day. During 17-18 May the JMA reported that ash plumes rose to an altitude of 1.8 km (6,000 ft) a.s.l. and drifted E and SE.

According to a news article, a small explosion at Aso on 14 April emitted a plume of "white smoke" to ~200 m above the crater (~5,900 ft a.s.l.) and deposited ash around the crater. The explosion occurred after hundreds of small earthquakes were recorded by JMA during the previous 2 weeks.

According to the Japanese Meteorological Agency, a "mud eruption" occurred at Aso's Crater 1 on 14 January at 1541. The eruption was accompanied by volcanic tremor and ash emissions that rose to low levels above the crater. Small amounts of very fine ash fell in Takamori Town about 10 km ESE of the crater. The level of thermal activity at Aso had risen during the previous year, with the last "mud eruption" occurring in July 2003. The Alert Level at Aso was raised from 2 to 3, and no tourists were permitted entrance within 1 km of the crater.

During 12-14 July, JMA recorded seismic signals at Aso that were associated with five small phreatic eruptions. Around 1400 on 27 July continuous volcanic tremor started. Such activity had not been recorded at Aso since November 1995. As of 28 July ~10 earthquakes occurred per day, and around 100 isolated tremor events had occurred since 23 July. On the 28th the crater lake in Crater 1 was gray, 76 °C, and bubbling in the center.

A tremor event with a moderate amplitude was recorded at Aso on 10 July at 1718. Aso weather station personnel inspected the area around Nakadake crater and found a small amount of tephra newly deposited at Hakoishi-Toge about 6 km ENE of the crater. Dr. Yasuaki Sudo of Aso Volcanological Laboratory, Kyoto University, inspected the crater area and determined that a phreatic eruption had occurred. Mud emitted during the eruption reached as far as 10 km from the crater. The color of the crater lake surface changed to dark gray from green, its color on 8 July.

By 21 August isolated tremor events that began at Aso's Crater 1 on 5 August decreased in number after peaking on 15 August. On the 21st the temperature of the crater's southern inner rim was still high (314 ºC), as it had been the previous week.

During 5 August to at least 15 August, isolated volcanic-tremor events occurred at Naka-dake, Aso's historically-active intra-caldera cone. The maximum number of events (335) was recorded on 12 August. Scientists found that the temperature of the southern crater wall remained high (307 ºC on 14 August) as it has since April 2002. There were no changes in water-pool temperature in the crater, nor had changes occurred in water level, sediment content, or fumarolic activity in the crater. The last time over 300 isolated volcanic-tremor events per day had been recorded at Aso was during 19 June-2 July 1992.

Information is preliminary and subject to change. All times are local (unless otherwise noted)

April 1970 (CSLP 39-70)

Ash eruptions from a new vent at Crater 1 of the Naka-dake cone

Card 0925 (30 April 1970) Ash eruptions from a new vent at Crater 1 of the Naka-dake cone

The Aso Volcanic Observatory reported that Mt. Aso has become active for the first time in four years. Volcanic ash and gases erupted more than 150 m into the air from a new opening inside of the No. 1 crater on the Nakadake volcanic cone of Mt. Aso. At first, a small amount of ash erupted from a little gap located in the NNE corner of the crater's bottom on 21 April. On 22 April a depression about 20 m long and 10 m wide was observed in the same place which formed a new crater. It spewed gray ash into the air reaching the brim of the outer crater 150 m above it. An eruption of this size is the first of its kind since May 1966.

Normal fumarolic activity in [Crater 1] began to increase on 31 March. Slight rumbling on 4 April was succeeded on 11 April by heavy rumbling, further increase in gas emission, and ejection of some grayish ash. On 12 April, the grayish [plume] contained many fist-sized cinders, which fell on the floor of Naka-dake's crater. Glow was seen 21 April but the [plume] had turned white and contained only a little ash. There was slight [ash] emission from the crater bottom, which had contained a small hot water pool.

Emission of gas and tephra from the [Crater 1] of Naka-dake continued through June. Much ash and sand, and occasional fist- to hand-size cinders were ejected. Activity declined in early July, but an explosion on 20 July at 1321 sent a column of ash . . . about [1,500] m above the crater and [ejecta lightly] injured [three] persons nearby.

The increased activity on 31 March was accompanied by continuous large amplitude tremor. A small vent formed on 8 May and emitted ash. During the night, a 2-10 m "flame" was observed. In late May, more fist-sized ejecta fell on the crater floor. An earthquake swarm occurred 3 June, and large-amplitude tremor, some of which could be felt near the crater, was recorded 18 June. Small scale eruptions deposited ash in early June. These eruptions increased in strength 20-23 June, depositing fist-sized ejecta inside the crater.

Activity then declined until 20 July, when explosions at 1321, accompanied by airshocks, projected black and gray ash clouds 1,500 m above the crater. Ash fell up to 500 m from the crater, reaching a maximum depth of 30 cm, and blocks up to 80 cm across fell on the rim. Similar explosions occurred at 1341 on 22 July, producing a 1,300 m cloud and depositing considerable ash around the crater. Activity then declined. No fresh magma was ejected at any time during the eruption.

A steam and ash cloud rose more than 300 m from Naka-dake at 0704 on 8 November. Ash ejections continued through the end of November, producing substantial ashfalls in and near the crater on 14 and 18 November.

Increased activity from [Crater 1] of Naka-dake continued through April. The vapor cloud contained little or no ash during March, but on 4 April the emissions increased in volume and a small ashfall was observed. Continuous emission of a grayish-white cloud began 7 April and lasted through the end of the month. Small quantities of mud and fine particles were ejected from vents in Naka-dake crater, but none of this material rose more than 20 m above the crater bottom. None of the ejecta contained evidence of fresh magma. Short-period volcanic tremor recorded by seismographs near the crater continued through April.

Steam and ash emission from [Crater 1] of Naka-dake occurred 29 May-1 June, producing an ashfall on the N side of the crater. Rumbling accompanying the activity could be heard from [JMA's Asosan Weather Station] 1 km from [the crater of] Naka-dake. Mud and [block] spattering was observed in mid-June, but the ejecta rose only 40-50 m and remained within the crater. No further activity has been reported.

An explosive eruption from Naka-dake crater began at 1510 on 13 June. Activity lasted more than 1 hour, producing a 1,500-2,000-m-high steam and ash column, and thundering sounds. [Blocks] larger than a man's head were thrown 400 m above the crater rim. Kyodo radio reported that the eruption reintensified during the night of 15-16 June. Hot tephra was ejected to 200 m above the crater rim, accompanied by a roaring noise. Doors and windows rattled in nearby houses and some residents fled the area, according to police reports. . . .

A small explosion occurred at the beginning of June from Crater 1 of Naka-dake (figure 1). Activity then declined for 10 days, although an arthquake was felt on [9] June. Loud rumbling and frequent small explosions egan on 12 June and incandescent block ejection was seen that night. At 1510 on the 13th, a larger explosion produced a 2-km-high ash column and the ejection of numerous blocks. Lightning could be seen in the ash column. Ash emission as observed until nightfall and incandescent blocks continued to be ejected every few minutes through the night of 13-14 June. A field investigation on 14 June revealed more than 10 cm of ash and many scoria bombs and blocks (up to 70 cm in diameter) in the summit area.

Figure 1. Map of the summit area of Naka-dake at Aso. Naka-dake's crater includes smaller craters 1-7 (hachured lines). Vents 741 (active since 1974) and 791 (formed during the June activity) are shown in crater 1. The dashed line delineates the area where tephra larger than 3 cm in diameter fell during June, 1979. Courtesy of JMA.

Frequent periods of activity, consisting of weak and continuous ash emission or explosive block ejection, occurred daily until 25 June. Incandescent coria was seen at night on the 14th, 15th, and 16th. A new vent (791 pit) was formed during the June eruption at the bottom of Crater 1 near 741 pit, which had been active since 1974 (figure 1). Since the end of June, activity has declined to occasional weak ash emission. Seismicity remained low in June, in contrast to the strongly increased seismicity that accompanied the eruptions of 1977 (figure 2).

Figure 2. Number of recorded seismic events per 10-day period at Aso, January 1976-March 1980. Arrows represent eruptions. [Originally from 05:04.] Courtesy of JMA.

Mid-June eruption successfully predicted, but activity continues, killing two in September

"The eruption of Aso continued through July (table 1). Seismicity remained low. It is empirically known for Aso volcano that the amplitude of continuous tremor becomes large before an eruption and remains large throughout the eruptive period (figure 3).

Table 1. Activity at Aso, late May-29 September 1979. Ash thicknesses, at about 1 km from the source, were estimated by weighing the small amount of material accumulated in a measured area. Courtesy of JMA.

Figure 3. Aso seismicity since 1973. Monthly number of isolated tremor events (top) plus 10-day means of continuous tremor anplitudes during the same period (bottom). Arrows show months in which Aso was erupting. Courtesy of JMA.

The local disaster control group for Aso volcano [closed the area within 1 km of the crater] at 1310 on 11 June because high-amplitude continuous tremor had begun to be recorded at JMA's Asosan Weather Station [originally referred to as Aso Observatory] during the early morning. Civil Defense personnel kept people 1 km from the crater, visited by many persons when the volcano is inactive. The eruption began during the evening of 12 June. No casualties have occurred.

"The second characteristic event of this eruption was the decrease in the amplitude of the continuous tremor just before the largest explosion, on 13 June. The extraordinary decrease in amplitude was observed for 11 and 1/2 hours, from 0336 to 1457 on the 13th. The explosion occurred at 1510, after a steep increase in tremor amplitude for 13 minutes. Many cases of a decrease in tremor amplitude before a larger explosion are known for past eruptions at Izu-Oshima and Aso. For example, a decrease lasting four days was recorded before Aso's large explosion of 31 October 1965."

Kyodo radio reported that [three] persons were killed and [11] injured by blocks ejected at about 1300 on 6 September. The area within 1 km of the active vent remained off limits.

"Eruptive activity continued through early September. Ash eruptions occurred almost every day from mid-July until 10 August (table 1) and ash fell on towns near the volcano; ash reached Takeda City, 30 km NE of Aso, in early August. Strong rumbling resumed on 10 August, but the volcano suddenly stopped emitting ash at about 1300 that day. The rumbling lasted until 26 August and was occasionally heard at the towns of Aso-machi and Ichino-miya-machi, 10 km from the crater. A steep decrease in the amplitude of the recorded continuous tremor took place at about 0900 on 27 August and the volcano was very quiet (no ash or block ejection, nor any rumbling) until 6 September.

"A loud explosion occurred at 1306 on 6 September. A dark ash cloud, in which lightning was seen, rose 700 m. The air shock reached 0.8 millibars and the ground shock had an amplitude of 17 µm at the JMA's Aso Observatory, 1.2 km from the crater. Three tourists were killed, two injured seriously, and nine slightly, by falling blocks 10-20 cm in diameter at a site 0.9 km from the vent (figure 4). Numerous blocks pierced the roof of a ropeway station (also 0.9 km from the vent), made of concrete as thick as 25 cm. A few of the lesser injuries occurred inside the station house. People said that the blocks that fell around the station house were hot and the cores of some of them were dimly glowing. Ash reached Oita city, 65 km NNE of Aso. The activity declined to white vapor emission 7 minutes after the explosion and no further eruption had occurred as of 12 September.

Figure 4. Summit area of Aso. Casualities from the 6 September 1979 explosion took place at the site marked X. Cinders larger than 3 cm fell in the area enclosed by a dashed line, a maximum of 1,200 m from the vent, Crater 1 of Naka-dake. Craters 1-7 are shown within the larger Naka-dake Crater, by hachured lines. Courtesy of JMA.

"The amplitude of recorded continuous tremor remained small (about 2 µm) through the explosion, became large (to 17 µm) 40 minutes after the explosion, then declined gradually to around 5 µm 14 hours after the explosion. It is not known whether the low-amplitude stage, which lasted from 27 August to just after the explosion, was an example of `amplitude decrease prior to explosion' (4:8). Seismicity remained relatively low both before and after the explosion.

"The restricted area [designated] on 11 June by the local disaster control committee for Aso volcano was still [closed] on 6 September. The committee will reexamine the size of the restricted area (within 1 km of the crater), although the casualties occurred in this area.

"The summit area of Naka-dake was surveyed by JMA personnel and by Koji Ono of the Japan Geological Survey 8-11 September. Cinders larger than 3 cm were scattered in the area enclosed by a dashed line in figure 4, reaching 1.2 km from Crater 1. A large block found 0.3 km from Crater 1 was 4.6 x 2.6 x 2.6 m and weighed about 50 metric tons. No scoria or other essential fragments were found. The explosion is considered to be a steam explosion, and may have been caused by heavy rainfalls on 27 August (161 mm) and 3-4 September (127 mm)."

Further Reference. Wada, T., Kikuchi, S., and Ono, H., 1980, The explosion of Naka-dake, volcano Aso on the 6th of September, 1979: Bulletin of the Volcanological Society of Japan, v. 25, p. 245-253.

After the large 6 September eruption that killed three persons, Aso remained quiet through 22 September. Occasional weak ash emission took place 23-29 September (table 1). All of the eruptive activity occurred from the 6 September vent.

Frequent ash ejections resumed on 24 September and continued through late November. During October, no blocks were seen to reach the rim of Naka-dake Crater nor were incandescent blocks observed at night. By the end of October, the concentration of ash at the JMA's [Asosan Weather Station] (1 km from the active vent) had reached more than 10 kg/m2, equal to about 1 cm of ash thickness. Although continuous tremor amplitude had correlated well with June-September eruptive activity, amplitudes remained low (about 0.5 µm) during October. The number of local earthquakes also remained low in October.

A characteristic decrease in the amplitude of continuous tremor began at about 0900 on 2 November, lasting until a large explosion at 1626. An eruption cloud rose 1.5 km above the crater during about an hour of ash ejection. Four mm of ash fell at the [Weather Station]. A survey by [Weather Station] personnel two days later found scoria up to 200 m from the vent, overlying 0.6 m of ash that had fallen in the summit area since the eruption began 12 June. The tremor amplitude decrease was the third since June that had preceded a sizeable explosion. An alert was issued from the Observatory one and a half hours before the explosion. No casualties occurred.

Ash emission in November was stronger than in October, causing heavy ashfalls near the volcano. Slight ashfalls occurred occasionally at Mt. Takachiho (110 km S), Kumamoto city (40 km W), and in Oita Prefecture (50 km E). Ejection of incandescent blocks was observed at night on 11 and 19 November, for the first time since 6 August. Tremor amplitude increased through most of November, but declined late in the month. The Strombolian activity of June, July, and early August occurred while tremor amplitude was high.

Activity stopped on 28 November after strong ash emission through most of the month (table 2). Between June and November, ash caused about 1 billion yen ($4 million) in damage to crops and forests. No further ashfalls were observed until an explosion at 2107 on 26 January deposited 3 cm of ash and fist-sized scoria on the rim of Naka-dake, the source crater. A small amount of ash fell on Aso-machi town, at the base of the volcano. The explosion caused no damage, and the volcano returned to quiescence the next day.

A weak and brief emission of ash from Aso occurred on 8 March, producing ashfall on the S flank. Since strong ash emission stopped on 28 November 1979, eruptive activity had been confined to ejection of a small amount of ash and scoria, accompanied by a strong air and ground shock, on 26 January, and emission of white vapor at other times.

The amplitude of continuous tremor recorded at [Asosan Weather Station] declined in December 1979 and has remained low through March (figure 5). The number of local earthquakes increased somewhat around the January tephra ejection, but declined in February and did not increase substantially during the March activity.

Figure 5. Daily mean amplitude of continuous tremor (bottom) and occurrence of ash emission (top) observed from the JMA's Aso-san Weather Station, May 1979-March 1980. Larger ashfalls are represented by longer lines in the top section. The arrows in the bottom section indicate strong explosions on 13 June, 6 September (three persons killed), 2 November 1979, and 26 January 1980. Courtesy of JMA.

Activity at Aso has been confined to weak but steady emission of white vapor since the brief 8 March ash ejection. The number of seismic events per day and the amplitude of the continuous tremor recorded at [Asosan Weather Station] were both small in April and early May.

A brief, weak explosion on 24 September ejected ash to about 800 m above Crater 1 of Naka-dake [after quiescence since the 8 March ash ejection]. The area within 1.5 km of the summit was closed immediately after the explosion but reopened 2 days later.

Ash and block ejection from Crater 1 of Naka-dake, the northernmost of 7 in Naka-dake, was observed at 1230-1300 on 15 June, after 9 months of quiescence. Blocks rose to 30 m, but fell within the 100 m-diameter crater. 1 µm ground shocks were recorded at 1239 and 1244, and a 3.7 µm shock at 1251 [at the Weather Station]. Activity then subsided. The explosions caused no damage. The area within 1 km of the summit, closed immediately after activity began, was reopened 17 June. Asosan Weather Station personnel [visited the crater on 15 June and] observed that the greenish water pooled in Crater 1 since October had become gray tinted [but returned to its usual green the next day]. The [level of some points on the surface of the water] rose intermittently. Naka-dake is the historically active part of the Aso volcanic complex.

A warning of increased volcanic activity was issued by the [Asosan Weather Station] on 11 October. On the morning of 24 October, ash was ejected from a fumarole that had formed in mid-September on the lowest part of the E inner wall of [Crater 1 of] Naka-dake. A small gray plume rose to 300 m above the fumarole at 0920, but the ejection was too weak to send ash beyond the crater rim. Entry to the area within 1 km of the crater was prohibited shortly before 1000. Another plume rose to 200 m at 1030 the next day, accompanied by intermittent ejection of small blocks, as large as fist size. Activity had subsided by 1550.

The level of water in Crater 1 has gradually decreased since early April (figure 6). A part of the crater bottom could be seen in mid-September. As of 26 October, only 30% of the bottom was covered by the hot water.

Ash was ejected a little more vigorously on 31 October, rising to the crater rim (about 100 m above the vent). On 2 November, ashfall was again confined to the crater. On 5-6 November weak rumbling was heard at the JMA's Aso Weather Station, 1.2 km SW of the crater.

Occasional weak ash emission that continued after 2 November was interrupted by water flowing into the active vent on 12 November. From then until 30 December muddy water and small rocks were ejected almost continuously, at varying intensity. During the strongest activity, on 9 December, this mixture fountained to an average height of 10 m above the crater floor, sometimes to more than 20 m.

Moderate ejections of ash to 150 m (as high as the crater rim) resumed 30 December and ended 2 January. On 30 December at 0815 an ash-laden plume rose 200 m above the rim. Ash fell on the S slope of the crater . . . . A prohibited zone within 1 km of Crater 1 was established 28 November.

On 11 January a grayish plume was observed rising 400-500 m above the crater rim, where ashfall was 1 cm thick. A little ash also fell on the S flank. Since the 11th an ash-laden plume was observed almost every day in January. Activity increased slightly on 18 January, when a gray plume rose 600 m above the rim. Volcanic flame [from a pit on the crater floor] was observed [during visits] the nights of 21 and 25 January.

Moderate ash-laden emissions from Naka-dake were occasionally observed in February and March. Gray plumes were seen on 8, 17-18, and 25-28 February. Ash accumulation on 26 February was 295 g/m at JMA's [Asosan Weather Station] (at the SW foot of Naka-dake [1.2 km from the crater]). On 1 March, the emission from the pit that had been active from January through February declined rapidly, although a new pit formed about 40 m N of the older one. Ash emission resumed the next day. Volcanic flame [from the pit on the crater floor was observed during visits] the nights of 6 and 25 March.

In late December, the press reported that volcanic tremors were continuously felt. However, JMA noted that average tremor amplitude was within the range of 0.1-0.3 µm through December and January. As of 1 April, tremor remained at about the same level.

Moderate ash-laden emissions from Crater 1 of Naka-dake were observed on almost every day in April (some daily ash accumulations are listed in table 3). Volcanic flame had been observed rising 20-40 m above [the pit in] the floor of Crater 1 on 25 March, and the remainder of the water pool in the crater disappeared on 16 April. Average tremor amplitude remained at around 0.3 µm in April.

Table 3. April-May 1985 daily ash accumulation at the JMA's Aso-san Weather Station, 1.2 km SW of Crater no. 1 of Naka-dake. [Ash was measured each day at 0900 and represents the amount deposited the previous 24 hours. There was no accumulation on other days in April and May.]

On 6 May, a small eruption was observed at a new pit about 10 m E of the one that had formed on 1 March. Rocks several tens of centimeters in diameter rose to a height of 50 m above the floor of Crater 1 and ash-laden emission was almost continuous from the 1 March pit. It was not certain if juvenile material was included in the 6 May tephra.

Since October 1984, activity has gradually increased at Naka-dake, site of all of the more than 140 eruptions known from the Aso complex in historic time. Activity at Crater 1 increased in May. Ash accumulated at JMA's Asosan Weather Station on 12, 14, [16], 22, 23, 29, and 30 May (table 3). An ash-laden plume was observed almost daily (figure 7). On the morning of 6 May, incandescent blocks were ejected at a new vent that had not been present the previous day. Block ejection stopped on 8 May but resumed on 15 May, when the vent increased to 40 m diameter by collapse of a wall between it and another vent that had formed in March. Incandescent blocks, including scoria a few tens of centimeters in diameter, were ejected to 50 m above the crater floor. Ash and mud jetted to the level of the top of the wall that had separated the two vents.

Incandescent blocks were occasionally ejected at the vent after 16 May. The most powerful May eruption occurred on the morning of the 30th; larger amounts of incandescent blocks (mainly scoria) as much as 1-2 m in diameter were ejected 120-130 m above the crater floor at 1020. The intensity of ejection then gradually declined.

Activity at Aso has gradually increased since October 1984. Moderate ash emissions from Crater 1 of Naka-dake were observed almost daily from the beginning of May (10:6) until 20 June. White steam vapor dominated for the last 10 days of the month. Total June ashfall at JMA's [Asosan Weather Station] was 1429 g/m22.

Activity declined in July, when neither ash plumes nor deposits were observed at the [Weather Station]. On the morning of 1 July, [Weather Station] personnel found that the vent on the floor of the crater had been covered by water from heavy late-June rains. The level of water within the crater gradually rose in early July. About 70% of the crater floor has been covered by hot (60°C) water since mid-July. There were many fumaroles along the margin of the pool, and sand and water were ejected at many points on the surface. Seismic activity remained low throughout June and July.

The number of tremor episodes recorded by a seismograph 0.8 km from [Crater 1] increased sharply to ~5,900 in May from the typical 100-500/month (figure 8). Activity declined slightly beginning 30 May to about 100 tremor episodes/day by 6 June. Background tremor was continuous and amplitudes in May were twice usual values (figure 10). The floor of Naka-dake's crater 1 had been covered by a pool of water that gradually dried up toward the end of May. Mud ejection about 10 m high from the crater bottom was observed during a field survey 26-27 May. Public access to the area around the crater was prohibited 25 May-6 June.

On the night of 16 October, JMA geologists noted irregular areas of red glow in a 30 x 10 m area on Naka-dake's crater floor, the first glow seen there since June 1985. The local government prohibited access within 1 km of the crater 17-27 October. Steam emission was observed from Crater No. 1 and its small crater lake shrank. The lake temperature was 71°C on 1 October. Seismicity and tilt showed no significant changes.

Ash ejection from [Crater 1] was observed on 28 December at 1420, during a field survey. The ash cloud rose 30 m above the crater rim. Red glow at vents and cracks in the crater floor had often been seen since 16 October. The number of isolated tremor episodes, counted from data recorded on a seismometer 0.8 km W of the crater, had gradually increased since the end of October, although the amplitude of continuous tremor episodes remained almost the same. No ash ejection was observed during a 31 December field survey.

About 3,000 volcanic tremor episodes were recorded during March, twice the number recorded in January and February (figure 9). A significant increase was recorded 22 March on a seismometer 0.8 km W of Crater 1. The amplitude of continuous tremor was generally unchanged.

Early 5 April, ash was ejected to ~50 m above a vent ~100 m below the crater rim. A field survey at 1140 the next day revealed that ash emission had stopped. A 1-km area (the smallest of three designated zones) around the crater was closed to tourists by the Aso Disaster Prevention Authority, at 0920 on 5 March. Such a restriction is necessary a few times a year when activity increases.

On 12 April, the restricted zone was reopened. Ash had not been ejected since 6 April although the frequency of tremor episodes remained high at 200/day. Glow at vents and cracks on the crater floor was regularly observed during night visits from October 1988 through April 1989.

On 27 April, the staff of AWS visited the crater rim as they have every day for the past 20 years. A vent on the SE floor of Crater 1 was releasing yellow vapor and ash to 30 m, accompanied by larger tephra. The Aso Volcano Disaster Prevention Authority closed a 1-km area near the crater to tourists. The area was reopened 2 May, when a field survey revealed only white vapor reaching ~5-6 m above the vent.

Glow on the crater floor has been observed every night since October 1988. A maximum temperature of 232°C was measured (with a infrared radiation thermometer) at a glowing site on 18 April.

Isolated tremor remained frequent in April. The daily number of tremor episodes was 100-250, with a monthly total of ~5,760 (figure 10). Amplitude of continuous tremor remained the same.

After a small ash ejection 5 April, tephra emission continued at a relatively high rate in May and June. On 8 May at 1000, a vent (1 m in diameter) on the Naka-dake crater floor ejected ash to ~10 m. At 1132, an M 3.3 shock (3 on the JMA Intensity Scale) occurred beneath the crater and was felt at AWS. Five (felt) aftershocks were recorded on 8 May (at 1120, 1147, 1216, 1417, and 2039), and 1 (not felt) was recorded the next day (at 0057) by a seismograph 0.8 km W of the crater. A 1-km area around the crater was closed to tourists by the Aso Disaster Authority. During a field survey at 1910, no ash ejection was observed.

On 16 May, ash rose ~100 m above the crater rim at 0810, and ~200 m at 1030. About 20% of the crater floor was covered by a rainwater pool, from which mud and water were continuously ejected to 3 m. During a field survey on 20 May at 1150, a strong rumbling noise was audible, but no ash ejection was seen.

Ash rose ~200 m above the crater rim on 22 May from 0740 to 0800, and 20 m above the crater floor at 0820. Activity declined, stopping by 1000. Two days later at 1000, ash was ejected to 200 m above the crater rim, and 5 g/m2 of ash was deposited at AWS. Ash had not fallen there since 28 June 1985. Red glow at the vent and in cracks on the crater floor was observed at night through May. During the night of 27 May, red glow emanated from 40-50% of the crater floor. On 28 May, ash rose about 50 m from the N portion of the vent.

In June, a vent on the NW floor of Crater 1 emitted an ash-laden steam plume a few hundred meters above the crater rim. During a 6 June field survey, the vent had enlarged and was emitting a 300-m ash plume. Flames from burning volcanic gases were occasionally observed rising 3-4 m above the crater floor during night visits. Ash accumulation at AWS was 9 g/m2 on the 7th, and 2 g/m2 on the 8th. The Crater 1 vent was buried by ash during rainfall 8-9 June. A new vent (named "891") about 18 m in diameter opened in the center of the crater floor on 10 June, and was the largest new vent since "853" formed 6 May 1985. The highest plumes of the month reached 1,000 m above the crater rim on 7 and 20 June.

Isolated volcanic tremor remained high (200-400 events/day) in May and June (figure 11) with a total of 5,760 events in May and 6,752 in June (compared to 5,821 in April). The amplitude of continuous tremor was generally unchanged in May but increased slightly in June.

On 16 July, Aso erupted vigorously for the first time since May-June 1985, with three ash explosions from Crater 1 (at 1354, 1603, and 1625). The eruption resumed on 24 July, ejecting ash to 2,000 m above the crater rim.

Activity at Crater 1 has been gradually increasing since the end of 1988 (figure 12). Visits to the volcano October 1988-5 August 1989 revealed red glow at vents and cracks on the crater floor. After a small ash ejection on 5 April, tephra emission continued at a relatively high rate in May and June. A new vent (891) on the crater floor, first noticed during a field survey 11 June, emitted an ash-laden plume almost every day in July. Ash often fell near the vent, and daily accumulations of 7 g/m2 on 6 July, 11 g/m2 on 7 July, and 6 g/m2 on 8 July were measured at AWS.

On 14 July, the largest daily number of isolated volcanic tremor episodes (744) in 1989 was recorded by a seismometer 0.8 km W of Crater 1 (figure 13). The same day, at 1535, the amplitude of continuous tremor increased (figure 14), and white vapor and ash from Vent 891 was ejected to 1,200 m. A 1-km area around the crater was again closed to tourists by the Aso Disaster Prevention Authority. Two days later, at 1155, tremor amplitude began to increase, decreased sharply at 1303, but increased again at 1344, 10 minutes before the onset of ash emission. Ash reached about 2,500 m height at 1355. After a sharp decrease in tremor amplitude, two ash explosions followed at 1603 and 1625, ejecting a plume to ~1,000 m.

On 18-19 July, small amounts of ash were repeatedly ejected from Vent 891 after tremor amplitude sharply decreased. During a field survey at 1920 on 22 July, a maximum brightness temperature of 506°C was measured (by an infrared radiation thermometer) at a vent on the crater floor. The amplitude of continuous tremor began to increase 21 July, and significantly increased on the night of 23 July. Ash emission resumed at 1620 on 24 July and continued for about 1 hour, attaining a maximum height of 2,000 m above the crater rim. A small ash ejection was observed during a field survey on 29 July. Brightness temperatures near the vent on the crater floor were 504°C on the 29th and 498°C on the 30th. The average amplitude of continuous tremor remained high.

An ash ejection on 14 August at 1050 was the fifth since a series of explosions began on 16 July (14:07). Ash emission continued for about 90 minutes, sending a blackish-gray plume to ~1,000 m above the crater rim. Amplitude of continuous tremor during the eruption was about 0.6-0.7 µm (on a seismograph 0.8 km W of the crater), compared to an average amplitude of 0.3-0.4 µm. Intermittent ash ejection continued almost daily through August, with ashfall often observed around Crater 1. During the month, a total of 787 g/m2 of ash accumulated at AWS about 1 km SW of the crater. Ash emitted 20-24 August reached villages ~40 km NW of the crater, the most distant ash deposition since 1979. On 4 September, the amplitude of continuous tremor decreased for a few minutes, followed by ash ejections at 0905 (1,000-m plume), 1300 (700 m), 1540 (2500 m) and 1725 (2,500 m). A 1-km area around the crater has been closed to tourists by the Aso Disaster Authority since 9 August.

Burning gas from the crater floor's 891 vent was observed almost daily during August. The flame was 30 m high on the night of the 23rd, the highest since burning gases were first seen in June, and a 15-m flame was seen the next day.

A large eruption occurred on 20 April after 2 months of quiet. On 13 February, activity decreased to emission of white vapor and weak mud ejection from a water pool in [Vent 892]. Because of the decreased eruptive activity, the area within 1 km of the crater (closed since 9 August 1989) was reopened by the mayor of Aso town on 23 March.

At 1323 on 20 April, tremor amplitude suddenly increased by about five times. An "Extra Volcanic Information" was issued by the Aso town office and the tourist viewing area was closed by the mayor (both at 1415). Tremor amplitude abruptly decreased at 1644 and remained low until the eruption began, similar to several previous episodes. At the onset of the eruption at 1708, tremor amplitude again suddenly increased. Ash and block ejection was accompanied by a loud rumbling noise which was strongest at around 1730. Blocks 0.5 m across were ejected to 300 m above the crater rim throughout the eruption. The ash cloud rose more than 1 km, and three electrical discharges (lightning) were observed between 1808 and 1815. Tremor amplitude decreased at around 1815 and the eruption is believed to have ended shortly thereafter, although the exact timing is unknown.

Ash from the eruption, wet due to rain, fell mainly N of the volcano where it affected traffic and caused agricultural damage. The towns of Aso and Ichinomiya lost electricity at around 1800, presumably due to a short circuit or leak caused by the wet ash. Service was restored to most homes by midnight, although some had no power until noon the next day.

The eruption deposited 1 m of ash on the crater rim, 0.5 m of ash 1 km N of the crater, and 4 kg/m2 at 6 km N of the crater. Fine ashfall was observed to 35 km W (at Kumamoto) and 35 km E (at Takeda). Blocks, mostly dense and angular, and fresh scoria fell N of the volcano. Lithic clasts 20 cm in diameter were scattered to 1 km N. The total tephra mass from the 20 April activity was estimated to be 1 x 109 kg (the total accumulated tephra mass from July 1989 through 20 April 1990 was 6 x 109 kg; table 6).

Table 6. Number of days/month on which ash was emitted at Aso and monthly ash accumulation measured by JMA at AWS, 1 km W of the crater, July 1989-May 1990. The April 1990 value is low, despite the large emission on 20 April, because of wind direction.

Ash emission was almost continuous from the morning of 21 April through early June. Another vigorous eruptive episode occurred 27 April and such episodes became frequent at the end of May. Tremor remained at low amplitude for a few days following the 20 April eruption then gradually increased to high amplitudes in May.

Recorded explosions became more frequent in mid-October, occurring almost every day during the second half of the month (see table 4), for a total of 29 since 16 July. Minor ash emission occurred on most days in October, causing ashfalls around the crater. Ash reached AWS during favorable wind conditions, and by 21 October had accumulated to 3 cm depth (11,409 g/m2). On 27 October, 8437 g/m2 of ash was deposited at AWS, and slight ashfall was observed at Kumamoto Local Meteorological Observatory, ~35 km SW of Aso. The heavy ashfalls damaged agricultural products. A zone within 1 km of the crater remained closed to tourists by the Aso Disaster Authority.

Block ejection, to 10 m height, from Vent 891 was first seen during a 6 October field survey. During 9 October fieldwork, a new vent (named 892) about 1-2 m in diameter was observed about 10 m N of Vent 891, intermittently ejecting fist-sized blocks to 20 m height. Ash emission continued from 891 vent, but no blocks were ejected. The next day, Vent 892 had grown to ~5 m in diameter. By 21 October, the new vent had enlarged further to 25 m across, coalescing with 891 the following day.

Flames from burning gases rising several tens of meters from the vent were often seen at night. Strong rumbling was sometimes audible at AWS, and rumbling was heard 10 km to the ESE (at Takamori) during the 28 October eruptive episode.

The number of isolated tremor episodes increased toward the end of October (figure 16). The amplitude of continuous tremor, recorded by a seismograph 0.8 km W of Crater 1, grew particularly large from mid-October, reaching a maximum of 12.8 µm on the 21st.

Eruptive episodes have been recorded on 36 days since 16 July, including 11 days in November (see table 4). Minor ash emission, without recorded explosions, occurred on most days in November, causing ashfalls around the crater. The month's ash accumulation at AWS was 1,409 g/m2. The 23 November eruptive episode, accompanied by lightning, ejected blocks to 300 m above the crater rim; blocks had begun to be thrown over the rim as recorded explosions became more frequent in mid-October. During a 24 November field survey, fist-sized blocks were seen 700 m SSW of the crater. Fieldwork on 26 November revealed that the cone on the crater floor had disappeared and the wall between craters 1 and 2 had been removed. Felt shocks of intensity I (JMA Scale) occurred on 19 and 26 November, centered under the summit crater. The number of isolated volcanic tremor episodes and the amplitude of continuous tremor, recorded by a seismograph near AWS, remained large. Rumbling was audible every day at AAWS and was strong on 4 and 25 November.

Activity was less vigorous in December than in November, with only three recorded explosions (see table 4), bringing the year's total to 39. Minor ash emission was observed almost daily. A total of 755 g/m2 of ash was deposited at AWS. The number of isolated tremor episodes recorded by a seismometer at AWS decreased in late December (figure 17). The amplitude of continuous tremor declined to a level similar to that of early September (figures 17 and 18). Rumbling was slightly audible at AWS during the month.

Block and ash ejections increase in late January; daily ash emission in February

Activity was relatively quiet in the first half of January, but increased in the second half of the month. A 21 January explosion ejected blocks to 300 m above the crater rim. Additional explosions occurred at 1645 on 1 February and 1320 on 7 February, the latter continuously ejecting blocks to 300 m above the crater rim. Minor ash emission was observed almost daily, causing ashfalls around the crater. A total of 30 g/m2 of ash was deposited in January and 3,167 g/m2 in February at AWS. Volcanic tremor amplitude increased from the end of January, but declined toward the end of February.

A pool of water was present in Vent 892 during fieldwork on 15 February. Mud ejection was observed for the first time since September 1989. Vent 892 began to develop in October, and has gradually enlarged to occupy half of the crater floor.

A large eruption occurred on 20 April after 2 months of quiet. On 13 February, activity decreased to emission of white vapor and weak mud ejection from a water pool in [Vent 892]. Because of the decreased eruptive activity, the area within 1 km of the crater (closed since 9 August 1989) was reopened by the mayor of Aso town on 23 March.

At 1323 on 20 April, tremor amplitude suddenly increased by about five times. An "Extra Volcanic Information" was issued by the Aso town office and the tourist viewing area was closed by the mayor (both at 1415). Tremor amplitude abruptly decreased at 1644 and remained low until the eruption began, similar to several previous episodes. At the onset of the eruption at 1708, tremor amplitude again suddenly increased. Ash and block ejection was accompanied by a loud rumbling noise which was strongest at around 1730. Blocks 0.5 m across were ejected to 300 m above the crater rim throughout the eruption. The ash cloud rose more than 1 km, and three electrical discharges (lightning) were observed between 1808 and 1815. Tremor amplitude decreased at around 1815 and the eruption is believed to have ended shortly thereafter, although the exact timing is unknown.

Ash from the eruption, wet due to rain, fell mainly N of the volcano where it affected traffic and caused agricultural damage. The towns of Aso and Ichinomiya lost electricity at around 1800, presumably due to a short circuit or leak caused by the wet ash. Service was restored to most homes by midnight, although some had no power until noon the next day.

The eruption deposited 1 m of ash on the crater rim, 0.5 m of ash 1 km N of the crater, and 4 kg/m2 at 6 km N of the crater. Fine ashfall was observed to 35 km W (at Kumamoto) and 35 km E (at Takeda). Blocks, mostly dense and angular, and fresh scoria fell N of the volcano. Lithic clasts 20 cm in diameter were scattered to 1 km N. The total tephra mass from the 20 April activity was estimated to be 1 x 109 kg (the total accumulated tephra mass from July 1989 through 20 April 1990 was 6 x 109 kg; table 6).

Table 6. Number of days/month on which ash was emitted at Aso and monthly ash accumulation measured by JMA at AWS, 1 km W of the crater, July 1989-May 1990. The April 1990 value is low, despite the large emission on 20 April, because of wind direction.

Ash emission was almost continuous from the morning of 21 April through early June. Another vigorous eruptive episode occurred 27 April and such episodes became frequent at the end of May. Tremor remained at low amplitude for a few days following the 20 April eruption then gradually increased to high amplitudes in May.

Intermittent eruptive activity has continued since 16 July 1989 from Crater 1. Eruptive episodes on 3, 5, 6, 7, 12, 13, and 18 June ejected ash clouds and blocks and were similar to those of previous months. Ash ejection was most active on 13 June, as tephra rose continuously to ~1,000 m throughout the day and the month's highest ash cloud rose 3,000 m above the crater. The 18 June activity was also strong, ejecting blocks 300 m W from the center of the crater. Ash accumulation at AWS . . . was 9,713 g/m2 in June, slightly lower than the May total of 12,837 g/m2 that was the heaviest monthly ashfall since October 1989.

Tremor amplitude gradually increased through the end of June after the abrupt drop that followed a larger eruption on 20 April. Short-period and large tremors recorded between 22 June and 1 July were thought to have been generated by small phreatic explosions. The bottom of the crater was occupied by a water pool after heavy rains 14-16 June and 26 June-4 July. Tremor amplitude was very small 1-9 July. AWS issued an "Extra Volcanic Information" on 4 July, warning of the potential for a sudden explosion following the abrupt decrease in tremor amplitude.

. . . Ash clouds and blocks were ejected [from Crater 1] on 7 days in June 1990, most recently on the 18th. However, no eruptions occurred in July. Tremor amplitude increased from 22 June to 2 July, decreased, then gradually increased for the remainder of the month. The bottom of Crater 1 was covered with a pool of water.

Vent 892 on the NE floor of Crater 1 had been covered by a pool of water since the last noted ash ejection in the crater on 30 June. Frequent mud and water ejections, and white steam emissions occurred during July and August. A plume containing small amounts of ash, intermittently ejected to 100 m from a vent in the SW part of Crater 1, was noted along with strong rumbling during a visit 30 August. The number of tremor episodes gradually decreased toward the end of August and tremor amplitude was at low levels.

White steam emission from Crater 1 continued, punctuated by weak ash emissions on 1, 14, 16, 17, and 27 September. Plume heights ranged from a few hundred to 1,000 m above the crater. Vent 892, site of activity until June, was buried by mud and heavy rainfall in July. A visit to the crater on 17 September revealed the existence of a new vent (901), not visible the previous day, which ejected blocks and flame to 10 m. Scoria blocks were ejected to 30 m above the vent on 28 September, the first magmatic ejecta since 18 June (15:06). The amplitude and number of tremor episodes increased for several days around 20 September.

No ash was erupted during October . . . . Crater 1 . . . continued to emit white steam that rose to 900 m above the crater. Weak ash emission was observed on 13 November, and glow from vents on the crater bottom was seen during fieldwork that night. The amplitude and number of volcanic tremor episodes increased in late October, reaching levels similar to September's and continuing at those levels through early November.

Crater 1, active July 1989-June 1990 (table 7), weakly emitted ash on 12, 18-19, and 25-29 November; white steam was emitted steadily on other days. The highest plume observed in November reached 1,000 m above the crater. Ash had last been emitted on 17 September. The area within 1 km of the crater, which had reopened to tourists on 15 October, was closed on 12 November and remained closed in early December.

Glow from many points on the crater floor was observed during a night visit on 13 November, the first crater glow seen since June. Glow remained visible through early December. During 17 November fieldwork, incandescent scoria was being ejected to 30 m height from a small vent on the crater floor. Scoria ejection had last been observed in June. By the 24 November crater visit, a vent 10 m across had developed on the crater floor and was ejecting blocks to 5 m height. The vent was named 902, the second new vent of 1990 . . . . An infrared thermometer detected a maximum temperature of 811°C in the vent.

Ash emission became frequent in early December. An eruption on 4 December at 1410 ejected a 1200-m ash cloud, December's highest (as of the 14th), and similar activity occurred on 6, 7, 8, and 13 December. Vigorous ash emissions had last occurred in June. Ejections of blocks and scoria were also more frequent and higher (to 150 m) in early December. A visit on 6 December revealed that a new vent . . . had opened near 902.

The amplitude and number of volcanic tremor episodes has gradually increased since October and remained high through November (figure 19).

Steam and ash were emitted periodically throughout December, to a maximum height of 1,200 m, on 4 December. A crater visit on 6 December revealed a new vent (903), 30 m long and 10 m wide. The center of eruptive activity had moved to Vent 903 from Vent 902 . . . . The amplitude and number of tremor episodes had gradually increased since October, and remained high through early January 1991.

Crater 1 emitted ash on 8-9, 18, 20-21, 23, and 28-29 January. The highest cloud observed in January reached 700 m above the crater on the 29th. Continuous tremor amplitude, which had been high since October, declined toward the end of January.

Crater 1 weakly emitted ash on 5, 8, and 9 February, and steadily emitted steam to 100-200 m heights on other days, a decline from previous months. The highest steam emission of the month reached 300 m above the crater on the 7th. Continuous tremor amplitude, which had been high since October, has declined since the end of January. Activity continued at similar levels through early March.

Isolated volcanic tremor episodes began to increase in October 1991, reaching about 100 events/day by the end of May. The increase in seismic activity followed a period of quiet after the July 1989-December 1990 eruptive phase. Ejections of mud and water, the first since June 1991, were observed within the active crater lake . . . on 23 April. Similar ejections, to 5 m height, were observed on 27 April, 1 and 27 May, and 2 June. The lake's surface temperature has been increasing since March-May 1991 when it was 20-30°C, reaching ~70°C (measured by infrared thermometer) in May. Weak mud ejections have been common in the past, during the period between eruptive phases when the crater is normally occupied by a lake, but have not been observed during the lowest levels of activity.

Eruptions that occurred from Crater 1 during the night of 30 June-1 July were the first [strong explosions] since . . . December 1990. The daily number of isolated volcanic tremor episodes began to increase in October 1991, and had reached ~100/day by the end of May. Isolated tremor episodes rapidly became more frequent in late June, and the amplitude of continuous tremor also increased through the month.

Ejections of mud and water from the lake in Crater 1 were first noted on 23 April and were sporadically observed later in April and in May. The ejections became more vigorous in late June, increasing in height from 5 m on 24 June to 20 m on the 26th, 50 m on the 29th, and 150 m on the 30th. Surface temperatures of the lake water increased from around 20°C in May 1991 to 78°C in June 1992. Steam plumes also grew to 1,000 m height in late June.

Strong tremor episodes were recorded during the night of 30 June-1 July. During fieldwork at noon on 1 July, the crater was quiet, but many blocks to 0.8 m across had been scattered to 100 m from the crater's NE rim. The eruptions were not seen or heard, but seismic and air-vibration records suggested that they may have occurred at 2349 on 30 June and 0316 on 1 July.

Tremor decreased in early July, but remained at higher levels than in mid-June. Ejections of mud and water to heights of a few tens of meters occurred sporadically through early July, but no additional strong mud/water ejections or eruptions were reported.

Because of the increasing activity, the area within 1 km of the crater was closed to tourists on 24 June, and remained closed as of mid-July.

Blocks were ejected during the night of 30 June-1 July from Crater 1 for the first time since . . . December 1990. Vigorous steam emission followed for about 10 days, fed a plume to a maximum of 2 km height on 6 and 8 July, then gradually declined toward the end of the month (figure 20). Ejections of water, mud, and blocks that rose ~50 m above the surface of the crater lake were observed almost every day during July. The lake shrank rapidly in early July until it occupied only about 1/3 of the crater floor. The temperature of the lake surface (measured by infrared thermometer) reached 95°C on 4 July (figure 20), the highest since March 1991, but declined to around 60° by the end of the month. Isolated tremor episodes, which had peaked at ~2,000/day at the end of June, declined rapidly after the block ejection to 0-6/day (figure 20). The amplitude of post-eruption continuous tremor also declined (figure 21).

Similar activity continued through mid-August, with weak mud ejections from the lake, steady steam emissions to 1,000 m height, and low-level seismicity. The lake expanded again to cover all of the crater floor by 5 August because of inflow of groundwater, precipitation, and weaker ejection activity.

Heavy rains expanded the crater lake in early August but it shrank to cover only 20% of the crater floor later in the month as a result of daily ejections of mud, blocks, and water. Steam emissions were continuous, rising to 1 km on 10 and 24 August.

A strong eruption of water, blocks, and mud from Crater 1 at 1223 on 8 September ejected blocks to 200 m above the crater rim and a 2-km steam plume. Blocks up to 70 cm across were scattered to 300 m W of the crater rim. The eruption was similar in size to the previous eruption on 30 June-1 July, but eruption tremor of 30 µm amplitude was detected by a seismometer 800 m W of the crater, compared to the 9-µm amplitude recorded by the same instrument on 1 July. No changes in tremor activity were recorded before the eruption, although there was an increase in the amplitude of continuous tremor for 2 days after the eruption (figure 22). There were only 55 isolated tremor episodes in August compared to 1520 in July, with another 35 events recorded in September through the 15th.

An eruption from Crater 1 ejected blocks at 1223 on 8 September, the first such activity since a similar episode on 30 June-1 July. Another eruption at 1627 on 29 September scattered blocks 800 m SE and ejected a steam plume 2,000 m high. The number of blocks and the distance they fell from the crater were greater than for the eruptions of 8 September and 1 July. Eruption-tremor amplitude was 30.2 and 30.4 µm, respectively, for the September eruptive pisodes.

Steam was steadily emitted to a few hundred meters throughout September, and volcanic-tremor frequency was low. No anomalies in steam emission or tremor frequency were noted either before or after the eruptions. However, continuous-tremor amplitude increased for two days after the 8 September eruption. Weak ejections of mud, blocks, and water continued.

An area within 1 km of the crater has been closed to tourists since 24 August, and no damage was caused by the eruptions. Similar activity has continued through 14 October, but there have been no additional eruptions.

An eruption from Crater 1 occurred at 1340 on 26 October, the fourth this year and the first such activity since 29 September. A steam plume containing ash rose 2,500 m, and wet ash fell 1.5 km S of the crater. No blocks were jected. Eruption-tremor amplitude was 20 µm at the nearest seismometer, 0.8 km W of the crater (table 8).

Steam was steadily emitted to a few hundred meters throughout October, and volcanic-tremor frequency was low. No changes in steam emission, tremor frequency, or earthquakes were noted before or after the eruption. Weak ejections of mud, blocks, and water to 15 m height continued in the crater lake, which ccupies half of the crater floor. Similar activity has continued through 14 November, without additional eruptions. The area within 1 km of the crater has been closed to tourists since 24 August. No damage was caused by the eruption.

No eruptions have been observed since a brief episode on 26 October from Crater 1. Ejections of mud, blocks, and water to 30 m height continued n the crater lake through November. Steam was steadily emitted to 500 m, reaching 1 km on 27 November (figure 23). Volcanic tremor and earthquake activity ere low. The area within 1 km of the crater . . . was re-opened on 12 November.

Surface activity increased in December. Continuous low rumblings were heard beginning on 1 December, and on the 3rd, blocks were ejected to 200 m from the crater floor. An area of 1-km radius was again closed at 1400 on 3 December. Observations the following day revealed that a new vent (named 921) about 5 m across had developed in the central part of the crater floor, producing flames 10 m high and ejecting incandescent blocks to 5 m height. The ejections continued the next day, but activity was unconfirmed after 6 December. The continuous steam plume included minor ash 4-7 December but was white again on the 8th. The highest steam plume rose 1 km on 5 December but the plume was only a few hundred meters high after the 6th. Seismicity was relatively low, unchanged from November.

No eruptions have been observed since . . . 26 October . . . . Ejections of mud and water to 20 m height continued in the crater lake through December. Steam was steadily emitted to 500 m, reaching 1,000 m on 5 December. The steam plume contained minor ash 4-7 and 26-27 December. [Vent 921] emitted ash until 7 December, then became inactive and was no longer visible a few days later. Observations on 26 December, following an increase in rumbling the previous day, revealed that a new vent (named 922) about 30 m across had developed near the site of Vent 921, and was emitting ash and steam. Seismicity was relatively low, with tremor amplitude gradually increasing after the middle of the month. The area within 1 km of the crater, closed to the public on 3 December, was reopened on the 30th.

Field reports confirmed that by 1 January the lake in Crater 1 had dried up. Steam was steadily emitted to ~500 m, with the plume containing ash 13-14 and 17-29 January. A small eruption occurred in the crater on 21 and 22 January, ejecting many scoria blocks to 10-50 m heights from Vent 922. This was the first eruption since 26 October and the first scoria eruption since June 1992. . . . Ejecta fell within the crater, which is 400 m across and 150 m deep. The steam plume, containing ash, rose 1,000 m on the 21st and 1,500 m the 22nd. Seismicity has been relatively high since mid-December, but no significant change was detected before or after the eruption.

Activity continued at the same levels through early February, with steam emission to a few hundred meters, occasionally containing ash.

The eruptive phase . . . continued through February at Crater 1. At noon on 20 February, vent 922 ejected scoriae to 100 m above the vent, the first eruption observed during daily visits to the crater rim since 22 January. The ejecta, consisting of blocks up to 50 cm across, fell within the 400-m-wide, 150-m-deep crater. Incandescent fountaining, also to a height of 100 m, was observed that night. Inclement weather prevented visual observations in the following days and the ending date for the eruption is not known.

The amplitude of the continuous tremor, which increased in late December, persisted at an elevated level until 10 February, when it rapidly declined (figure 24). The decline was not accompanied by any observed change in surface phenomena. At 0600 on 20 February, the tremor amplitude increased. Tremor amplitude remained high during and after the eruption, abruptly returning to background level at 1215 on 25 February after a small block ejection. An area within 1 km of the crater has been closed since 18 January.

Figure 24. Daily mean amplitude of volcanic tremor (top) and height of steam plume (bottom) at Aso from 1 January 1992 to 8 March 1993. Arrows at the top indicate eruptions. Courtesy of JMA.

Ash was observed in the steam plume on 1-5, 12-13, 18-20, and 23-25 February. The plume rose to heights of 500 m, increasing briefly to 1,000 m n 19-20 February (figure 25).

Activity in March and early April was lower than in previous months. Rain created a small lake in part of Crater 1. On roughly half of the March visits, mud and blocks were seen being ejected a few meters above the lake. A white steam plume continually rose 200-500 m; it contained a minor amount of ash on 7 March. Seismic activity was low.

Activity at [Crater 1] has been moderate since an explosion on 20 February 1993 ejected scoriae 100 m above the vent. During the daily rim visit on 2 May 1994, mud ejection was observed for the first time since 10 June 1993. However, the crater floor has been covered by water and frequent water ejections have been observed. Continuous tremor was registered at a seismic station 800 m W of the crater. Average amplitude of continuous tremor had been 0.2 µm through May, but on 7-9 June the average amplitude suddenly increased to >6 µm.

Crater 1 remained restless through June after a mud ejection on 2 May. The floor of the crater has been covered by a pool of water, but frequent water ejections have been noted during daily observations from the crater rim. Continuous tremor was registered at the seismic station 800 m W of the crater. During May, average tremor amplitude was around 0.2 µm. However, in early June, the amplitude increased suddenly. Continuous tremor became intermittent from 7 to 21 June, and isolated tremor occurred with a maximum amplitude >6 µm.

Activity from Crater 1 was moderate in August. However, at about 0800 on 11 September, intermittent mud ejection from the water-covered crater floor was detected seismically. Tremor registered at a station 800 m W of the crater had an amplitude of 4.8 µm. The seismic station detected similar activity on the evening of 12 September. During the daily crater visit on the morning of 14 September, several tens of stones were found outside the crater rim, within ~300 m of the crater center.

Activity increased at Crater 1 during September. Tremor amplitude registered at a seismic station 800 m W of the crater was 4.8 µm at about 0800 on 11 September. Three hours later, the AWS (figure 25), issued a Volcanic Advisory noting that Aso was getting restless. Another tremor, which was large enough to be felt at AWS, occurred at 1148 later that day. The floor of Crater 1 was covered by a pool of water, and intermittent mud ejection took place. Several tens of volcanic stones were found outside of the crater rim within ~300 m from the center of the crater during a visit on the morning of 14 September. These rocks were ejected by an explosion on the evening of 12 September, based on seismic records. The area within 1 km of Crater 1 was placed off-limits on 11 September by local governments through the Board for Volcanic Disaster Reduction.

During the rest of September, mud ejection was intermittent and volcanic tremor was frequent. On 15 and 18 September, ejected mud rose 150 m above the bottom of the crater, almost to the crater rim. On 16 and 19 September, a plume rose to a height of 1,500 m above the crater rim. Tremor was felt by personnel at AWS on 11, 15, 21, 22, and 29 September, and 1 October. The 29 September event was registered 800 m W of the crater with an amplitude of 52 µm, which is the largest reading since tremor amplitude measurements began in 1969.

The 12 September ejection of stones beyond the crater rim was the first eruptive activity since February 1993; mud ejections have been reported since 2 May 1994.

Figure 26. Seismicity and plume heights at Aso, January-October 1994. Earthquakes and tremor were registered at a station 0.8 km W of Nakadake cone. Plume heights were estimated by personnel at AWS. Courtesy of JMA.

During November, no eruptive activity took place at Crater 1. Water and gas ejection from a pool of water on the crater floor was observed on 5 days in November (specifically, 2, 3, 6, 27 and 28 November). Tremor amplitude registered at a seismic station 800 m W of the crater was not greater than 0.5 µm, but in December the amplitude began to rise.

Intermittent mud ejection from the pool of water on the floor of Crater 1 continued in December. Ejections on 18 and 23 December rose 150 m, almost to the level of the crater rim. Large-amplitude tremor was often registered, and 12 seismic events were felt at the AWS.

Intermittent mud ejections from the lake in Crater 1 continued through mid-January. Mud ejections that rose 100 m on 2 January were accompanied by tremor, seven seismic events felt at the AWS, and a white plume rising to 2 km above the crater rim. At other times during January a white plume rose continuously to ~400 m above the crater rim from the water-covered crater floor, ~150 m below the rim. Large-amplitude tremor associated with the mud ejections was often registered, and one other event was felt on 4 January.

Mud and water ejections continued during February from the shrinking pool of hot water in Naka-dake Crater 1. Similar ejections occurred on 13 and 17 March. The eruption on 17 March ejected mud and volcaniclastic materials within a 300-m radius, including some beyond the crater rim, and sent an ash cloud as high as 1 km above the crater rim. Large-amplitude tremor associated with the mud ejections was felt at the Aso Weather Station (AWS) on 14 and 19 February, and another nine times during March. An earthquake centered beneath the crater was also felt at AWS on 16 February.

During April and May, occasional water ejections took place from a hot water pool at the bottom of Naka-dake Crater 1. Water volume in the pool had decreased by 60% in late May. On 9 April mud and water ejections were observed at the bottom of the crater, in addition to a large-amplitude tremor felt at the Aso Weather Station. The daily number of isolated (short-duration) tremors increased in the middle of April, and during May a total of 1,128 were recorded from Station A, 800 m W of Crater 1.

Water rises and covers the crater floor, minor water and mud ejections

During June, occasional water ejections took place from a hot water pool at the bottom of Naka-dake Crater 1. The volume of water in the crater increased towards the end of June such that by July Naka-dake's crater was completely covered with hot water. During July, the occasional water ejections were accompanied by the ejection of mud, the highest reaching 10 m.

In July, 791 isolated tremors were recorded at Station A, 800 m W of Crater 1. Continuous tremor occurred through early July, with a maximum amplitude of 8 µm. There were seven natural tremors during July, including four felt at the Aso Weather Station and three earthquakes. Only one large-amplitude tremor was recorded during June.

The bottom of Naka-dake Crater 1 remained covered with a pool of hot water throughout August. The central part of the lake was gray, changing to grayish white or green near the margins. Mud and water ejections were frequently observed; the highest rose 10 m. Isolated tremors increased late in the month (recorded 800 m W of the crater). Isolated tremor events totalled 2,613 during August, and five earthquakes were detected. Tremor events continued increasing in early September; by the 10th there had been >2,000 counted.

Throughout September the hot water pool on the floor of Naka-dake Crater 1 frequently ejected mud and water; the highest ejection rose 10 m. Many isolated tremors were recorded at Station A, 800 m W of the crater. The monthly total of isolated tremors was 6,618; only two earthquakes were detected. Continuous tremor with 0.2-0.8 µm amplitude was registered throughout the month.

Mud ejections have been reported since May 1994 (BGVN 19:05). The 24-km-wide Aso Caldera contains 15 central cones. One of these cones, Naka-dake, has erupted more than 165 times since 553 AD, the first documented historical eruption in Japan. Aso is located 75 km E of Unzen and 150 km N of Sakura-jima volcanoes.

During October the floor of Aso's active crater (Naka-dake Crater 1) remained covered by a pond of hot water. The pond's surface was disrupted by occasional fountaining up to 5-m high. Elevated tremor continued since last month, and some October days had over 200 earthquakes; the daily mean amplitude of continuous tremors sometimes reached over 0.5 þm. Personnel 800 m W of the crater (at Aso Weather Station) felt earthquakes at 1829 and 1909 on 11 and 22 October, respectively.

During November and December 1995 the floor of Naka-dake Crater 1 remained covered with hot water, yet there were few if any mud-and-water ejections. During November the number of isolated tremors reached 5,488; during December, 4,896. In addition, continuous tremor prevailed with amplitudes confined to 0.1-0.8 µm.

The floor of Naka-dake Crater 1 remained covered with water in January. Seismic station A, 800 m W of the crater, recorded continuous tremor of 0.1-0.3 µm amplitude. In addition, there were 4,966 isolated tremors during the month.

Aso, a 24-km-wide caldera, produced pyroclastic-flow deposits during the Pleistocene that cover much of Kyushu. Its frequently active Naka-dake is one of a group of 15 intra-caldera cones.

Red glow has been observed over part of the S wall of Naka-dake Crater 1 since 27 April. The floor of this crater was still covered with water in May. Aso, a 24-km wide caldera, produced pyroclastic-flow deposits during the Pleistocene that cover much of Kyushu. Naka-dake, one of the 15 intra-caldera cones of Aso's caldera, has erupted more than 165 times since 553 AD.

Red glows were observed over the S wall of Naka-dake Crater 1 in May and June. The crater floor was covered with water in June, and weak water ejections were observed on 5-6 June. On 30 July, Crater 1 ejected mud on its SE side. Such ejections were also observed on 26 October 1995 (BGVN 20:10).

The 24-km-wide Aso Caldera contains 15 central cones. One of these cones, Naka-dake, has erupted more than 165 times since 553 A.D.

Tomoki Tsutsui (Aso Volcanological Laboratory, Kyoto University) reported that a new fumarolic vent ~10 m in diameter formed on the S wall of Crater 1 in early November; later, small mounds of mud formed around the vent. Although Crater 1 had been quiet since 1993, hot greenish-gray water remained in the crater. Videos of Crater 1 taken by the Aso Volcano Museum recorded emissions of mud fragments and white fumes from the new vent, as well as a bubbling noise; other instruments detected low-level volcanic tremors.

According to news reports, inhalation of volcanic gas killed two men, aged 62 and 51 years, after they collapsed ~100 m S of Crater 1's rim at 0945 and 1040 on 23 November. Volcanic gas concentration around the crater is monitored using a sensor installed by the Japan Meteorological Agency in April 1997. Due to high levels of SO2 (~5 ppm), the Crater 1 overlook was closed on the morning of 23 November, but re-opened at 0900 when the SO2 level dropped to2 levels rose to ~8 ppm. The weather station at Aso had recorded no abnormal volcanic conditions.

Seventy-one people have been hospitalized due to inhalation of volcanic gases at Aso since 1980; of those, seven were killed. In June 1994, five junior high school students on a field trip collapsed near Crater 1.

Aso, a 24-km wide caldera, produced Pleistocene pyroclastic-flow deposits that cover much of Kyushu. Fifteen central cones form an E-W line on the caldera floor. Naka-dake, one of the intra-caldera cones, has erupted more than 165 times since 553 AD. Naka-dake has a group of craters (1.1 km long) including Crater 1 at the summit. Strombolian, phreatic, and phreatomagmatic eruptions are common in Crater 1. The 4 km2 100-m-deep Crater 1 is accessible by cable car, automobile, and on foot.

Recent noteworthy activity at Aso consisted of elevated tremor in August 2002 and a phreatic eruption in July 2003. Seismicity recorded by the Japan Meteorological Agency (JMA) between January 2000 and April 2003 (table 1) was generally constant, with continuous volcanic tremor every month in addition to isolated tremor events. The number of tremor events was high through October 2000, during April 2002, and from August 2002 through March 2003. Also during this extended period, white plumes were observed approximately once a month, with two or more plumes occurring in July, October, and December 2002, and February and March 2003. These plumes were usually less than 500 m high.

Table 1. Seismicity at Aso between January 2000 and April 2003. The seismograph station is located ~ 13 km W of the summit. Courtesy of JMA.

Activity during August 2002. For the first time since 1992, isolated volcanic tremor events occurred at a rate of more than 300 events/day in Naka-dake Crater 1. These events were recorded between 5 and 21 August and totalled nearly 4,000 (table 2), with the highest number, 340 events, on 15 August. During this period, the water temperature of the pool in the crater remained between 57 and 60°C. On 14 August, infrared cameras measured the maximum temperature of the southern crater wall at 307°C; this increased to 314°C the following week.

Activity during July 2003. JMA reported on 11 July 2003 that tephra had fallen at Aso that morning. According to the report, a tremor event with an intermediate amplitude was recorded at 1718 on 10 July. Staff from the Aso Weather Station confirmed that small amounts of tephra had been newly deposited at Hakoishi-Toge (Hakoishi Pass), ~ 6 km ENE of the Nakadake crater. Kazunori Watanabe (Kumamoto University) and other geologists surveyed the deposit on 11 July and estimated the total mass of ejected material at roughly 130 tons. Ash was deposited as far as 14 km from the crater. A small amount of fresh vesicular glass particles were noted in the ejecta under the microscope. According to Yasuaki Sudo (Aso Volcanological Laboratory, Kyoto University), who inspected the crater area, the event was a small phreatic eruption of mud. The deposit consisted of wet ash aggregates and was ~ 1 mm thick, even at the crater rim. A spray of mud was blown off the crater rim by strong winds to 10 km from the crater.

Seismic signals implied a series of small phreatic eruptions between 12 and 14 July. Then on 27 July continuous volcanic tremor started around 1400. Observations that day noted that the water in Crater 1 was gray and boiling in the center; the temperature of the water was 76°C.

Japan Meteorological Agency (JMA) reports for July 2003 noted ash-bearing eruptions from Aso (BGVN 28:10). The 10 or 11 July 2003 eruption was followed by seismically inferred phreatic eruptions a few days later, and a mud eruption on 14 January (the end of this report interval).

Seismic signals during 12-14 July implied there had been about five small phreatic eruptions. Continuous volcanic tremor started at ~ 1400 on 27 July. The JMA report of 28 July 2003 noted that seismometers had recorded ~ 100 isolated tremors. Earthquakes also occurred, at a rate of ~ 10 per day. On 28 July, lake water in Crater 1 was gray colored with a temperature of 76°C and with boiling regions in its central area.

A later JMA report noted a mud eruption in Crater 1 at 1541on 14 January, the first such eruption since July 2003. Associated tremor also occurred. Small amounts of very fine ash from this eruption were seen in Takamori, a town ~ 10 km ESE of the crater. The report noted that thermal activity had risen since last year, causing the water level in the crater to decrease about 40% below normal. The hazard status rose from 2 to 3, and accordingly, authorities restricted tourists from the area within 1 km of the crater.

Small ash-bearing eruptions during May and to lesser extent in June 2011

After small ash-bearing eruptions, the Alert Level on Aso was raised from 1 to 2 (on a scale of 1-5) on 17 May 2011. Aso, the largest volcano in SW Japan, consists of a large, 24-km-diameter caldera located on the Japanese island of Kyushu (figure 27). Under normal conditions the area within the caldera is restricted and, with the raising of the Alert Level, authorities restricted entry within 1 km of Naka-dake cone, containing one of the active craters that comprise the Aso volcanic complex.

Figure 27. Map of the main islands of Japan; Mt. Aso is on the Island of Kyushu. Map from Wordtravels.com.

Fumarole temperature from hydrogen isotopic ratios. The temperature of fumarole gas is a primary observation used at many volcanoes. Tsunogai and others (2011) describe a method of using hydrogen isotope ratios to determine fumarolic temperature. Because the isotopes can be collected at a distance from the vent and without entering the crater, this method offers several advantages. Aso was one of the volcanoes where this technique was applied because it has a deep crater that makes direct sampling and at-vent measurements impractical. Direct sampling of gases is potentially far more hazardous. Infrared measurements may suffer bias when cooled, outgassed material, such as ash, obstructs the hotter portions of the plume. Such measurements could understate the emission's radiant heat and thus its temperature.

We previously published a topographic map depicting the Aso caldera and the location of Naka-dake within the caldera (BGVN 19:09 ), one of 17 central cones. Of these, Naka-dake is the most active. Naka-dake has a crater lake at its summit that contributes to its tendency towards phreatic and mud eruptions.

Aso resides in a National Park of the same name. Naka-dake is easily accessible by public transport and is a popular tourist destination. The rims of the active crater area contain parking and viewpoints accessible by toll road or the Arcosan Ropeway (steel-cabled aerial tramway). At the rim, massive concrete structures offer some protection from falling ballistics in the case of sudden explosions. Another attraction in the area is the Aso Volcano Museum, which features a webcam and photos of phreatic eruptions at Aso.

Aso has been highly active in recent years, but rarely to an extent where it has become dangerous to people. Aso erupted from 10 June 2003 to 14 Jan 2004 (BGVN 29:01). During that time Aso mainly erupted mud, associated with volcanic tremors, and a small amount of ash. A rise in thermal activity in the area may have been a contributing factor in the eruption (Volcano Research Center, University Tokyo). On 14 April 2005, the volcano erupted again, forcing five tourists to be evacuated after hundreds of small earthquakes were detected in the prior two weeks.

May-June 2011 unrest. The latest series of eruptions began on 6 May 2011 with mud erupting about 5-10 m from the hot caldera lake. On 13 May the temperature of fumarolic emissions in the caldera had increased. The Japan Meteorological Agency noted that "A small volcanic flame [glow?] has been observed at nights at the crater pits in the center of Naka-dake." On 15 May, Naka-dake erupted a small amount of ash, with the plume rising to an altitude of 2.1 km. One approach to measuring areas of elevated temperature involves infrared photographs. Visible and infrared photos of Naka-dake crater documented temperature increases in the crater from 21 April to 15 May (figure 28). The temperature of fumarolic emissions in the crater reached around 370°C (the temperature measurement method was not disclosed).

Figure 28. Visible (left) and infrared (right) images on two different days (contact JMA for temperature scales). a) 21 April 2011 and b) 15 May 2011. Courtesy of JMA.

On 16 May, an eruption sent one plume to an altitude of 1.8-2.1 km and another ash plume to 2.4 km, according to the Tokyo Volcanic Ash Advisory Center. A video depicting the plume that day can be found on Youtube (Asahi.com, 2011). The video shows aerial footage of the plume, which is bent downwind. The emission is constant but not vigorous.

Rocks ejected from Naka-dake on 17 May landed in restricted areas, and ash plumes rose to an altitude of 1.8 km. Ash plumes continued rising to similar altitudes through the end of May, and small scale eruptions continued through 10 June, accompanied by low level seismicity. No additional plumes were reported through mid-October. The website of the Mt. Aso Ropeway noted that entry restrictions ended on 20 June 2011, allowing them to carry passengers.

This report summarizes Japan Meteorological Agency (JMA) monthly reports (available in English since October 2010) covering the interval April 2011 to September 2012, with a separate subsection largely focused on aviation reports of Aso plumes emitted at Naka-dake crater during mid-2011. During this reporting interval Naka-dake continued to degas and emit small ash plumes. Eruptions of mud resumed after a hiatus of several years (February 2008 to April 2011).

Aso (also called Aso-san) is a caldera with dimensions ~17 km E-W by ~25 km N-S encompassing an area of ~350 km2. Figure 29 indicates the location of Aso in relation to other Holocene Japanese volcanoes and landmarks in the region.

Figure 29. A map of the major volcanoes of Japan. Aso is shown on the left side, on the island of Kyushu. Courtesy of the U.S. Geological Survey.

Aso’s most recent series of eruptions began in April 2011, with minor phreatic (mud-bearing) eruptions from Naka-dake’s crater lake. These eruptions were accompanied by minor ash plumes, rock ejections, an increase in the temperature of fumaroles (BGVN 36:09), and continuous, small-amplitude tremor.

Field observations during April 2011-June 2011. In April 2011, a small phreatic (mud-bearing) eruption 5-10-m-high was observed in Naka-dake’s crater lake; the lake’s temperature was 67°C. Volcanic seismicity remained at a relatively low level. A photo from 21 April 2011 shows a white steam plume (figure 30A).

Figure 30. (A) A photo taken by a field survey team on 21 April 2011 shows a white steam plume rising from the crater floor. (B) A photo taken on 16 May 2011 shows a grayish plume venting from the crater floor. Courtesy of JMA.

From 3 to 10 May, continuous small-amplitude tremor was detected. Seismicity, including isolated-pulse events, remained relatively low during this time. On 6 and 9 May, field surveyers observed a small 5-10-m-high phreatic eruption from the hot crater lake (locally called “Yudamari”).

A camera installed by the Aso Volcano Museum detected a small volcanic ash emissions from within the crater beginning on 13 May. Six cameras provide live image feeds to the Aso Museum website. There are also many videos showing Aso and Naka-dake on YouTube.

On 13 May, a field survey found increased fumarole temperatures in the crater, and a video camera revealed incandescence on multiple nights. According to JMA, a small eruption occurred on 15 May followed by minor ashfall, which extended 2 km NE of the crater. A field survey on 15 May recorded a temperature of ~370°C at a fumarole in the crater.

Another eruption occurred on 16 May, producing a grayish plume that rose 500 m above the crater rim. As a result of this increased activity, the Alert Level was raised from 1 to 2 (on a scale from 1-5). A field surveyer later the same day saw a gray plume rise 800 m above the crater rim (figure 30B). Small-scale eruptions occurred intermittently on the 17th. The lake water volume was low around this time, ~10-20% of its full volume.

A 9 June field survey revealed a decrease in fumarole temperatures from ~370°C on 15 May to ~160°C on 9 June. After 10 June, eruptions ceased and the lake water volume increased from 60% full on 12 June to 80% full on 17 June (figure 31). The rising lake level suggested a decrease in activity. Consequently, the Alert Level was lowered from 2 to 1 on 20 June. Seismicity, including isolated-pulse events, remained at relatively low levels.

Figure 31. (A) Photo taken on 9 June 2011 showing the bottom of Naka-dake crater. Note the absence (or near absence) of the crater’s lake. (B) Photo taken on 22 June 2011 showing the presence of the steaming crater lake just about two weeks after the photo in (A) was taken. Courtesy of JMA.

Plume heights and drift directions during May-June 2011. We summarize reports from the Tokyo Volcanic Ash Advisory Center (VAAC) issued between 15 May and 9 June 2011 (table 3). Many plumes contained ash. Notice that the plume heights are stated as altitudes above sea level (as compared to heights above the crater rim, as in the other sections of this report).

Table 3. Summary of plumes at Aso between 15 May and 9 June 2011. Smaller plumes may not have been recorded or were omitted. In most cases, the presence of ash in the plume was noted; in other cases ash may have been present but not recorded. ‘-’ indicates data not reported. Data provided by Tokyo VAAC and JMA.

Field observations during October 2011-June 2012. In October 2011, white plumes rose on average less than 200 m above the crater rim, with a maximum of 300 m. The lake water volume during September and October was at about 90% full, and the September and October lake-surface temperatures were 47-56°C and 49-58°C, respectively. Based on field surveys made on 3, 17, and 20 October, the sulfur-dioxide (SO2) flux was ~300-500 tons/day, compared to ~300 tons/day in September. Volcanic seismicity remained low. Tremor, detected 13 times during September, was absent during October. The total magnetic intensity measured at the NW rim of the Naka-dake crater had increased since December 2010, but was static during June 2011 through October 2011. No change was detected by GPS measurements.

The next JMA monthly report on Aso discussed activity during May and June 2012. Because of heavy rains after 15 May, the lake water volume had increased to ~70% full, and during the course of the month the volume was in the range 60-80% full. Then in late May, the lake level begain to drop, and continued into at least mid-June.

The lake surface temperature was 63-72°C in May and 67-73°C in June. The highest temperature of fumaroles along the southern crater wall was 246-260°C, compared to 228-267°C in May. Scientists conducting a field survey at night on 22 June noted that part of the S crater wall was incandescent.

In June 2012, white plumes rose an average of 600 m above the crater rim. There were 621 isolated cases of tremor in June, approaching a 2-fold increase over some of the previous months, but only amounting to a duration of a few minutes per month. Isolated volcanic tremor and seismicity remained low but had slightly increased overall after February 2012, with most hypocenters located at shallow depths under Naka-dake. No change was detected by GPS measurements. The total magnetic intensity began to increase again in June 2012.

Lake levels during July-September 2012. In July, heavy rains caused the lake level to rise to 80-90% full (from 30-70% full in June). The volume remained high in August and September (90-100% full). During June-July the lake surface temperature decreased slowly, from 58-66°C in July to 57-61°C in August and to 54-59°C in September. Steam emissions from the crater occurred in July and August, but stopped by September.

Crater temperatures during July-September 2012. The highest temperature of the S wall of Naka-dake-Daiichi crater decreased in July, but rose slightly in August and September (213-250°C in July, 241-249°C in August, and 250-283°C in September). A field survey on 24 September revealed that the hot areas had not changed since the previous survey on 22 June. On 23-26 September, weak glow in the crater was recorded at night by a thermal camera. Officials assumed the glow was caused by the hot crater wall.

July-September 2012 seismicity. Both isolated volcanic tremor and other seismicity returned at low levels during July-September 2012. 621 volcanic tremors occurred in June, 669 in July, 1,025 in August and 867 in September. 669 volcanic earthquakes occurred in July, 951 in August, and 978 in September. Other seismic events occurred 369 times in June, 626 in July, and were not reported in August or September. Few short-term tremors occurred (4 in June, none in July, 2 in August, and 1 in September). Most hypocenters were located at shallow depths (2-4 km) and in an area ~6 km NE of Naka-dake.

Based on field studies, sulfur dioxide levels were elevated during May-September 2012 (600-800 t/d in May, ~400 t/d on 10 July, and 500-700 t/d on 19 and 24 September). The total magnetic intensity at the NW rim of Naka-dake-Daiishi crater increased between December 2010 and September 2012, which officials suggested might signify a temperature rise underneath the crater.

Ongoing emissions leading to a larger, late-2014 eruption with ashfall

This report summarizes behavior at Naka-dake (Nakadake) crater at Asosan (Aso, Aso-san) caldera chiefly during January 2014-February 2015. During this reporting interval Naka-dake continued to emit gas, steam, and small ash plumes. A larger eruption took place starting 25 November 2014, causing ashfall and glowing emissions. This closed a local airport, and triggered hundreds of reports on ash plumes for the aviation community by the Tokyo Volcanic Ash Advisory Center (VAAC). That eruption continued through 2014. The eruption went on into 2015 but was generally described as intermittent during late December 2014 through at least the end of February 2015. The Alert Level remained at 2 (on a scale increasing from 1 to 5) for the duration of the reporting interval. Our last report, BGVN 37:08, described the emission of ash plumes and other behavior during May-June 2011. Some remarks in this report also refer to earlier behavior, for example, a short subsection includes what JMA recorded as important in a terse summary of 2011.

Eruption details were extracted and synthesized mainly from Japan Meteorological Agency (JMA) sources. JMA frequently communicated with the Tokyo VAAC about Asosan’s eruptive status. This report also discusses Volcano Ash Advisories (VAAs) issued by the JMA’s Tokyo VAAC. For many of the VAAs, evidence of ash-bearing plumes reported by JMA could not be reliably detected in the satellite images. For example, the images were sometimes obscured by overlying weather-cloud cover. The plumes were also generally only rising to a few kilometers in altitude. In at least some cases, the low plumes appeared bent by high winds.

Naka-daka Crater Number 1 remained the active vent for the most part during the past eight decades. That same pattern held true during this reporting interval when myriad small eruptions, often to or below 1 km above the crater rim were documented. Visibility was sometimes impaired but monitoring instrumentation confirmed a pattern of ongoing eruption. In some cases, the eruption was not clearly seen but fresh ash was recorded. Webcameras regularly documented incandescence both in the crater and onto the crater rim. Smaller ash plumes were too numerous to mention except in occasional cases. High winds were often mentioned, which may have bearing on restricting plume heights.

Location and brief background. Asosan is located on the S of the main island of Japan (Honshu) on the island of Kyushu (figure 32).

The rim of Nake-dake is unusually developed for such an active volcano. Both a road and cable car carry tourists there. Shelter dugouts are provided around the crater. The Aso Volcano Museum is located nearby.

Figure 33, made from radar imagery, shows Asosan’s morphology.

Figure 33. Morphology of the Asosan caldera seen in shaded relief (color scale for elevations absent). N is towards the top; the N-S cross-caldera distance is ~25 km. Note the central highlands, a series of ~17 post-caldera cones that includes the active Nake-dake and its Crater Number 1. The caldera’s topographic boundary is distinct on all sides but a drainage has breached the W side. This caldera vented four sets of massive eruptive deposits (pyroclastic-flows and associated ash-falls; Miyabuchi, 2013; Fujii, 2001). Source: Wikipedia (from a Nasa Shuttle Radar Topography Mission).

JMA (2013) includes a map showing the location of 12 calderas in Japan. Asosan, the largest and most active, has had many small eruptions in the past few thousand years, including many witnessed eruptions in the interval of recorded history. Spica (2013) discusses Aso in the context of other calderas in the Kyushu region.

Figure 34 shows a shaded-relief map focusing on the post-caldera cones in the central highland area.

Figure 34. A shaded relief map of the elevated central area (post-caldera cones and their craters) of the Aso caldera, adding naming labels in English of some of the main features. As seen in the previous figure, the caldera floor (moat) is outside and encircling this central topographic high. Both conventional topographic and a digital elevation map (50 m grid) were used to make this map, which was published by the Geospatial Information Authority of Japan. Source: JMA (2013).

JMA’s website features this summary on Asosan.

“Asosan (Aso Volcano) comprises the Aso caldera and post-caldera central cones. The Aso caldera, 25 km north-south and 18 km east-west in diameter, was formed by four gigantic pyroclastic-flow eruptions from approximately 270,000 to 90,000 years ago. Post-caldera central cones were initiated soon after the last caldera-forming eruption, producing not only local lava flows but also voluminous tephra layers which fell far beyond the caldera. Nakadake Volcano, which is the only active central cone of basaltic andesite to basalt [composition], is one of the most active volcanoes in Japan. The active crater of Nakadake Volcano is a composite of seven craterlets aligned N-S [elongate zone of depressions to the left of the label “Nakadake” and above the letter ‘t’ in the label “Nakadake-Crater” on figure 34; see also SEAN 04:07 for a sketch map focused on this area)]. Only the northernmost [Nakadake] crater (No. 1 crater) has been active in the past 80 years, although some others were active before the 1933 eruption. The Nakadake No. 1 crater is occupied by a hyperacidic crater lake during its calm periods. During active periods, its volcanic activity is characterized by ash and strombolian eruptions and phreatic or phreatomagmatic explosions.”

According to Fujii and others (2001), “Aso caldera in central Kyushu, Japan, is one of the largest calderas in the world and covers an area of 380 km2. In late Pleistocene time, eruptions of voluminous pyroclastic flows occurred intermittently, resulting in formation of the caldera. The Aso pyroclastic-flow deposits are divided into four major units, i.e. Aso-1, Aso-2, Aso-3, and Aso-4 . . . [and] welded tuffs of these units are widely distributed in central Kyushu, and are generally well suited for paleomagnetic research . . .. K-Ar ages for Aso-1, Aso-2, Aso-3, and Aso-4 have been determined to be 266 ± 14 ka, 141 ± 5 ka, 123 ± 6 ka, and 89 ± 7 ka, respectively (Matsumoto and others, 1991).”

JMA summary for 2011 activity. JMA (2013) tabulated a summary of witnessed events (eruptions, possible eruptions, damage, significant behavior, etc.) at Asosan going back to the year 553. In the most recent behavior discussed, the authors briefly note that during 2011 (an interval they term Heisei 23) the following behavior occurred.

First, after the Mw ~9 Tohoku earthquake ~70 km off the Pacific coast on 11 March 2011, earthquakes temporarily increased roughly 10 km to the NW of the active crater. Second, very small emissions of gray-white volcanic ash occurred during 15 May to 9 June 2011. On 15 May a very small amount of tephra fall was confirmed at Sensuikyo, ~2 km to the NE of the Nakadake Number 1 crater.

2014 Activity. JMA reporting for 13 January 2014 noted the emission of a very small eruption. This came in the wake of increased tremor in late December 2013 and an increase in hazard status to Alert Level 2. (As previously mentioned, the Level remained at 2 for the duration of the reporting interval.) Further escalation in tremor took place on 2 January. On 10 January emissions reached 1,200 metric tons/day (t/d) of sulfur-dioxide (SO2). The 13 January 2014 eruption took place at Naka-dake, which emitted a grayish-white plume that rose to 600 m that traveled S and deposited traces of ash. The resulting report from the Tokyo VAAC (a Volcanic Ash Advisory (VAA)) stating they failed to detect identifiable ash in the plume data captured on satellite images.

The small 13 January 2014 eruption triggered the first Asosan VAA in over a year. The other VAAs during January 2014 were issued on the 27th, 29th, 30th, and 31st. On one of those days, two VAAs were issued, and thus, for January there were 6 VAAs.

Bulletin editors note that the VAAs are not a linear measure of the number of eruptions. Small eruptions may not trigger a VAA at all. Several consecutive VAAs may occur associated with a single potentially larger eruption, which are issued in an effort to track an ash plume. Again, this may be an example where the number of VAAs is not reflective of the number of eruptions. Despite this, the number of VAAs are easily counted owing to new online archives. The Tokyo VAACs online presentation system is tablular in nature and is thus well suited to enable a count of reports per month.

The tally for VAAs on Asosan during 2013 was zero. The tally for 2014 involved 171 VAAs. Monthly totals for 2014 are as follows: January, 6; February, 3; March-July, 0; August, 3; September, 2; October, 0; November, 25; and December, 132. For further comparison, the tally for January and February 2015 involved 250 VAAs, with January, 132, and February, 118.

JMA reported that seismicity increased from 21 to 23 January 2014, and then decreased on 24 January. On 23 January a volcanologist observed ash plumes rising from the central vent on the crater floor. On 29 January an ash plume reported by a pilot rose to 2.7 km altitude and drifted NW. Later that day a plume rose to an altitude of 1.5 km and drifted N. JMA reported that a very small Asosan explosion occurred on 31 January. An off-white plume rose 100 m above the crater rim and drifted S.

According to the Tokyo VAAC during 30 August-1 September 2014 eruptions continuously emitted ash plumes that rose to heights of 1.2-2.1 km drifting N and NE. For example, on 1 and 6 September eruptions emitting trace amounts of ash sent plumes 600 m above the rim. (Tokyo VAAC issued VAAs stating this plume lacked identifiable ash in available satellite images.) JMA instrument surveys established SO2 flux rates on 21 August of 1,000 t/d, and in early September of 1,200 tons/day. Counts tallying daily volcano-tectonic earthquakes (and cases of tremor) were made during 1-4 September occurring in the range 48-92 (429-500); during 5-7 September occurring the in the range 55-129 (401-463); during 8-15 September occurring in the range 394-564 (80-174).

JMA reported that during 8-16 September a persistent white plume was observed 1 km above the crater.

Preliminary counts for volcanic earthquakes (394-564 per day) and tremor (80-174 per day) were reported during 8-15 September. Field surveys conducted on 9 and 12 September yielded elevated temperatures from fumaroles and the surface of the S crater wall.

Tremor accompanied a very small eruption recorded on 22-24 October. Ashfall observed on the 24th indicated another such eruption.

During 7 September and 24 November 2014, VAAs were absent for Aso. In contrast, during 25 November 2014-31 December 2014 there were 171 VAAs issued. Multiple VAAs were issued on several different days in this later interval, for example, on 26 November, 7 VAAs were issued.

Asosan continued to erupt during the 7 September-24 November 2014 interval. Some monitored parameters such as earthquakes, tremor, and SO2 emissions were elevated. A small eruption took place on 6 September, for example, sending a plume to 600 m above the crater. During 8-16 September JMA noted a persistent white plume 1 km above the crater. During the week 12-18 November, a steam plume rose 400 m above the crater rim.

With the start of the surge in VAAs beginning on 25 November 2014 (noted above), a stronger and comparatively sustained eruption began. During the eruption on the 25th an ash plume rose to 1.8 km above the crater rim. Ash soon fell to the E in Hanoi Aso (Kumamoto Region), Taketa (30 km NE, Oita Region), Gokase (25 km WSW, Miyazaki Region), and in Minamiaso (10 km SW, Kumamoto Region). Incandescence at night was seen with webcams.

On 26 November tephra ascended 100 m above the crater rim and an ash plume rose 1 km. Tremor began a few hours before the eruption and on the 26th, continued to be elevated. The eruption continued on 27 November; ash plumes rose 1.5 km. Volcanologists observed a strombolian eruption and found 7 cm of fresh ash that contained fist-sized scoria. Ash fell to the W, affecting the city of Kumamoto (38 km WSW). According to a news article, flights in and out of Kumamoto airport were either cancelled or diverted. On 28 November ash plumes rose 1.5 km. The eruption continued through at least 30 November; ash plumes rose at most 1.5 km and incandescent material was ejected onto the crater rim.

Although inclement weather restricted views of the crater, monitored parameters and available views indicated that the 25 November eruption continued through to at least 22 December, when it became intermittent. Ash plumes to about 1 km above the crater rim and incandescent material on the crater rim were common through the end of the year (and beyond, through this reporting interval ending in February 2015, and described as the ongoing eruption.

A news report in the 28 November 2014 issue of the Daily Mail by Sara Malm (Malm, 2014) indicated dozens of cancelled flights at Kumamoto’s airport. That report included the Associated Press photo seen in figure 35. The date of the photo in that article was ambiguous, but a different article with the same photo (see caption) gave 26 November 2014 as the photo’s date. The angled, bent-over character of the ash plume and location of Crater Number 1 (the active crater, at the N end of the row of craters) indicate the view was from the NW and implies strong winds roughly from the N.

Figure 35. A photo taken on 26 November 2014 of Asosan in eruption. The gray ash plume is escaping at Nake-dake Crater Number 1 blowing roughly S. The plume does not rise vertically. The plume ascends near the vent but for some distance beyond the vent the plume descends. At distance the plume appears to spread over considerable vertical extent, from near the ground surface to above the field of view. Source: Phys.Org news (crediting AP/Kyodo News).

An undated video in Malm (2014) also showed the plume. The video also showed an aerial view of the visitor area on the crater rim, which was blanketed in gray ash. Other scenes included children walking to school wearing dust masks and carrying folded umbrellas, and close up shots of what appeared to be dark colored, highly vesicular spatter.

In a 29 November 2014 MODIS image of the region, Asosan was under weather clouds but a clear view revealed a prominent ~30-km-long, beige-colored, funnel-shaped area trending SE. This was interpreted by Nasa Earth Observatory authors Jeff Schmaltz and Adam Voiland as airborne ash. Webcamera images around this time showed a glowing pit crater with extensive areas containing incandescent tephra around it. A copious plume also discharged nearby.

During a field survey on 10 December volcanologists observed 20-cm-wide blocks near the crater and 5- to 10-cm-wide blocks within 1.2 km SW of the crater. During 12-15 December the plume rose 1 km above the crater rim and ash fell to the E in Hanoi Aso (Kumamoto Region).

JMA reports for 15-30 December described the usual eruptive ash plumes that again rose 600-1,000 m above the crater and some cases of still glowing material on the crater rim. SO2 fluxes were 2,000-3,100 t/d during 15 and 18 December.

2015 activity. As noted above, the VAAs for 2014 totaled 171, and the VAAs for the months of January and February 2015 totaled 250. This is consistent with ongoing eruption at Asosan, which was also the basic conclusion in JMA reports from monitoring and direct observations during January-February 2015, although they often described the eruption during both these months as intermittent.

JMA reported cases during January where plumes rose up to 1 km above the crater, and in some cases glowing material reached the crater rim. JMA reported SO2 fluxes of 500-2600 tons of SO2. Both tilt and GPS instrumentation recorded slight growth across the active crater. A pilot report on 29 January indicated an ash plume to 2.7 km altitude (~1.1 km above the rim) and drifting NW.

An image acquired on 13 January 2015 was discussed by Jesse Allen and Adam Voiland of Nasa Earth Observatory. They reported that the image was from the Operational Land Imager (OLI) on Landsat 8. They indicated that it showed ash drifting ten’s of kilometers S from Aso.

For February 2015, JMA reported episodes of volcanic earthquakes, high-amplitude tremor, and infrasound data that continued to indicate ongoing intermittent eruptions. Webcamera views again documented cases of glowing material reaching the rim during the first half of the month. Plumes again rose up to 1 km above the crater rim. JMA reported intermittently detected eruptions, including during 2-6, 9-13, and 16-20 February.

Malm, S, 2014, Flights cancelled across Japanese region after Mount Aso volcano erupts for the first time in 22 years, spewing lava, smoke and a kilometre-high ash cloud, The Daily Mail 28 November 2014 (7 graphics files and a 58-second video) (accessed online June 2015) ((URL: http://www.dailymail.co.uk/news/article-2852674/Volcano-south-Japan-erupts-disrupting-flights.html#ixzz3d5POqZhu )

Volcano Types

Tectonic Setting

Rock Types

Population

Within 5 kmWithin 10 kmWithin 30 kmWithin 100 km

75,559
75,559
234,591
6,507,519

Geological Summary

The 24-km-wide Asosan caldera was formed during four major explosive eruptions from 300,000 to 90,000 years ago. These produced voluminous pyroclastic flows that covered much of Kyushu. The last of these, the Aso-4 eruption, produced more than 600 cu km of airfall tephra and pyroclastic-flow deposits. A group of 17 central cones was constructed in the middle of the caldera, one of which, Nakadake, is one of Japan's most active volcanoes. It was the location of Japan's first documented historical eruption in 553 AD. The Nakadake complex has remained active throughout the Holocene. Several other cones have been active during the Holocene, including the Kometsuka scoria cone as recently as about 210 CE. Historical eruptions have largely consisted of basaltic to basaltic-andesite ash emission with periodic strombolian and phreatomagmatic activity. The summit crater of Nakadake is accessible by toll road and cable car, and is one of Kyushu's most popular tourist destinations.

This compilation of synonyms and subsidiary features may not be comprehensive. Features are organized into four major categories: Cones, Craters, Domes, and Thermal Features. Synonyms of features appear indented below the primary name. In some cases additional feature type, elevation, or location details are provided.

Synonyms

Aso

Cones

Feature Name

Feature Type

Elevation

Latitude

Longitude

Akai

Pyroclastic cone

33 m

32° 46' 30" N

130° 49' 39" E

Eboshidake
Eboshi-dake

Stratovolcano

1337 m

32° 52' 0" N

131° 4' 0" E

Hontsuka

Cone

574 m

32° 57' 0" N

131° 4' 0" E

Ikenokubo

Tuff ring

Janoo
Zyanoo

Cone

754 m

32° 54' 0" N

131° 2' 0" E

Jigoku Spa

Vent

Kishimadake
Kizima-dake
Kishima-dake

Stratovolcano

1270 m

32° 53' 0" N

131° 4' 0" E

Komezuka
Kometsuka

Pyroclastic cone

954 m

32° 54' 0" N

131° 3' 0" E

Kusenrigahama

Pyroclastic cone

1160 m

32° 53' 0" N

131° 4' 0" E

Maruyama
Maru-yama

Stratovolcano

1186 m

32° 52' 0" N

131° 6' 0" E

Nakadake
Naka-dake

Stratovolcano

1500 m

32° 53' 0" N

131° 6' 0" E

Naraodake
Narao-dake

Stratovolcano

1331 m

32° 53' 0" N

131° 6' 0" E

Nekodake
Neko-dake

Stratovolcano

1408 m

32° 53' 0" N

131° 9' 0" E

Ojodake
Ozyo-dake
Ojo-dake

Stratovolcano

1238 m

32° 54' 0" N

131° 4' 0" E

Okamadoyama
Okamado-yama

Stratovolcano

1150 m

32° 51' 0" N

131° 4' 0" E

Omine

Pyroclastic cone

409 m

32° 49' 56" N

130° 55' 38" E

Takadake
Taka-dake

Stratovolcano

1592 m

32° 52' 51" N

131° 6' 23" E

Takanoobane

Cone

Washigamine

Stratovolcano

32° 53' 0" N

131° 7' 0" E

Yomineyama
Yomine-yama

Stratovolcano

913 m

32° 51' 0" N

131° 2' 0" E

Craters

Feature Name

Feature Type

Elevation

Latitude

Longitude

Aso

Caldera

Domes

Feature Name

Feature Type

Elevation

Latitude

Longitude

Yunotani

Dome

800 m

32° 53' 0" N

131° 2' 0" E

Thermal

Feature Name

Feature Type

Elevation

Latitude

Longitude

Akamizu

Thermal

440 m

32° 55' 0" N

130° 59' 0" E

Aso-Uchinomaki

Thermal

480 m

32° 58' 0" N

131° 3' 0" E

Jigoku

Thermal

740 m

32° 51' 0" N

131° 2' 0" E

Tarutama

Thermal

760 m

32° 52' 0" N

131° 2' 0" E

Yunotani

Thermal

800 m

32° 53' 0" N

131° 2' 0" E

Photo Gallery

Kometsuka scoria cone, on the NW side of the central cone complex of Aso volcano on the island of Kyushu, was constructed about 1800 years ago. Explosive eruptions building the cinder cone were accompanied by lava flows that traveled down the flank of this complex of stratovolcanoes in the center of Aso caldera. A broad moat separates the central cone complex from the caldera walls, which form the horizon.

Copyrighted photo by Dick Stoiber, 1981 (Dartmouth College).

Kometsuka scoria cone (lower left) was formed about 1800 years ago on the west flank of the central cone complex of Aso caldera. Clouds hover over a broad moat forming the caldera floor surrounding the cone complex. The NW wall of the 24-km-wide Aso caldera rises above the clouds. The caldera was formed during four major explosive eruptions during the Pleistocene between 300,000 and 80,000 years ago.

Photo by Ichio Moriya (Kanazawa University).

The 24-km-wide Aso caldera was formed during four major explosive eruptions during the Pleistocene. These produced voluminous pyroclastic flows that swept over much of Kyushu. A group of 17 central cones, seen here from the ENE caldera rim, was constructed in the central part of the caldera. One of these, Naka-dake, is one of Japan's most active volcanoes, with historical eruptions dating back to 553 AD. The summit crater of Naka-dake is accessible by toll road and cable car, and is one of Kyushu's most popular tourist destinations.

Photo by Norm Banks, 1981 (U.S. Geological Survey).

Steam rises from an incandescent vent on the floor of the crater of Naka-dake on April 25, 1985. Moderate eruptions that ejected mud and ash had begun in April 1984 and continued intermittently until June 1985. Frequent ash eruptions occurred from March 1 to the night of May 5-6, 1985.

Photo by Japan Meteorological Agency, 1985.

A gray eruption column rising above the crater of Aso volcano on October 11, 1989 is observed by tourists from a nearby parking lot. Minor ash ejection 30-50 m high began on April 5 and 27, 1989. Tephra emission remained at a high rate in May and June, and intermittent vigorous eruptions that began on July 16 were continuing in 1990. The ash column reached 3000 m on September 7 and 27, and heavy ashfall in October damaged crops. Intermittent eruptive activity continued until February 9, 1991.

Photo by Yukio Hayakawa, 1989 (Gunma University).

References

The following references have all been used during the compilation of data for this volcano, it is not a comprehensive bibliography. Discussion of another volcano or eruption (sometimes far from the one that is the subject of the manuscript) may produce a citation that is not at all apparent from the title.

Aoki K, 2008. Revised age and distribution of ca. 87 ka Aso-4 tephra based on new evidence from the northwest Pacific Ocean. Quat Internatl, 178: 100-118.

WOVOdat is a database of volcanic unrest; instrumentally and visually recorded changes in seismicity, ground deformation, gas emission, and other parameters from their normal baselines. It is sponsored by the World Organization of Volcano Observatories (WOVO) and presently hosted at the Earth Observatory of Singapore.

EarthChem develops and maintains databases, software, and services that support the preservation, discovery, access and analysis of geochemical data, and facilitate their integration with the broad array of other available earth science parameters. EarthChem is operated by a joint team of disciplinary scientists, data scientists, data managers and information technology developers who are part of the NSF-funded data facility Integrated Earth Data Applications (IEDA). IEDA is a collaborative effort of EarthChem and the Marine Geoscience Data System (MGDS).